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Dork Diaries: Tales from a Not-So-Happy Heartbreaker (also known as Holiday Heartbreak) is the 6th book in the Dork Diaries series. It was released on June 4th, 2013. 
Nikki is afraid to ask Brandon to the Sweetheart Dance. She is also afraid that before she gets a chance of asking him, he will agree on going with MacKenzie, the most popular girl of the school and Nikki's enemy. She gets a lot of mean texts from Brandon. Also, it was revealed in Tales From a Not-So-Smart Miss Know-It-All that she and her band, Dorkalicious, will record her original song, "Dorks Rule", with Trevor Chase. However, Trevor calls Nikki and tells her he won't be able to record with the band.
Saturday, February 1
Nikki is so happy because today, she is going to hang out with Brandon at Crazy Burger. She gets overcome with the desire to dance. After a dance session, she waits for hours for Brandon to call. But for some reason, he doesn't. Chloe and Zoey call and tell her that MacKenzie was at Brandon's party after she left, even though she wasn't invited. This makes Nikki worry about what MacKenzie could have done.
Sunday, February 2
Brandon still doesn't call Nikki. She believes he was abducted by aliens. She's also stuck babysitting Brianna, who is taking on a new persona called Miss Bri-Bri, Fashionista Hairstylist to the Stars. Brianna invites Nikki to her salon for a makeover. Nikki decides to play along and get a pretend makeover. Brianna gets straight to work while Nikki reads a magazine with a section telling how a girl can tell when a guy is not interested in her.
1. He agrees to a date and then rudely cancels at the very last minute.
2. He simply texts you an apology instead of telling you in person.
3. Suddenly he's always too busy to spend any time with you.
4. When you try to talk about your relationship, he just walks away.
5. He's spending way too much time with another girl.
While Nikki takes the article out and puts it in her pocket, she feels something. Brianna did something to her hair. Nikki demands a mirror and it is revealed that she tore a piece of Nikki's hair out. And to make matters worse, Brianna takes a photo of Nikki's new do with her phone. Nikki tries to get it back from her, but they accidentally send the picture to all the people on Nikki's contact list. Chloe and Zoey saw it and thought it was pretty funny. She's more worried about Brandon seeing it. And it just gets worse when she gets a text from Brandon, stating he was too busy to go to Crazy Burger. Nikki crosses the first two items off the list from the article and fears what could happen with her relationship with Brandon.
Monday, February 3
At school, all the guys are nervous and all the girls are in line. The annual Sweetheart Dance is coming up, and the girls ask out the guys. Jordyn tells Nikki to get a ticket before they sell out. Nikki, still uneasy about the Brandon situation, decides not to ask her. Of course, she knows all too well that MacKenzie will ask him. MacKenzie brags about it and tells her that she got him a camera for his birthday and tells her to vote for her for Sweetheart Princess on Valentine's Day. Nikki has no idea what's going on.
Tuesday, February 4
Even Chloe and Zoey are excited for the dance. In gym class, their practicing swim meets. Nikki says she's not going, so neither are her friends. She tries to convince them to go without her, but it was easier said than done. They argue for a little bit until class ends. Later, Nikki meets up with Brandon and refuses to let him tell her his side of the story for why he couldn't go to Crazy Burger. When she tries to talk about their relationship, he walks off. She crosses item 4 off the list. Later that night, she gets a bull dial from him, much to her frustration.
Wednesday, February 5
Nikki gets her band Actually, I'm Not Really Sure Yet together to perform their song Dorks Rule for their upcoming meeting with big producer Trevor Chase. As they practice, Nikki notices that Zoey is flirting with Theo and Chloe is flirting with Marcus. She thinks they'd be their perfect dates for the dance. She decides to get them tickets for the dance and one to ask Brandon if he's still available.
Thursday, February 6
Nikki arrives at school early to get tickets for the dance. Unfortunately, they're sold out. Because Nikki told her friends that she had a big surprise for them, she quickly looks into her locker and gives them the closest things she could find: a half-used tube of lip-gloss and a half-empty pack of tissues. They humbly accept them. Nikki later finds out that Trevor can't record their song because he's gonna be working on a TV special with Lady Gaga, so they'll have to reschedule the recording.
Friday, February 7
Nikki gets a note from her friends and believes they want to discipline her for the lame gifts she gave them. Instead, they reveal that they got her tickets for the dance. Nikki admits she wanted to do the same for them and is happy that they'll be going. Later, Brandon texts her and says he's busy at Fuzzy Friends. She crosses item 3 off the list.
Saturday, February 8
Nikki unwillingly takes Brianna to Kandy Kingdom. Mrs. Wallabanger, still hard of hearing within with her aid, arrives with her grandson Oliver, who is about the same age as Brianna and has a sock puppet named Mr. Buttons. Oliver and Brianna don't start off very well, but Mr. Buttons and Miss Penelope some to get along fine. Eventually, they all have a happy time together. As Oliver and his grandma leave, Nikki teases Brianna about a possible crush, which Brianna denies. Nikki can already picture the two of them being a cute couple.
Sunday, February 9
Nikki is not looking forward to the floating skills swim test at school tomorrow and imagines an excuse note to get out of it.
Monday, February 10
Nikki rummages through the old water toys and finds Brianna's old seahorse floaty, Sierra the Sea Horse. She decides to use it for her floating test but her teacher tells her she can't use it (even though the rules don't say so). However, when she tries to get it off, it sticks to her. She imagines her whole life wearing it until the janitor comes and pokes a hole it and lets all the air out, freeing her. Her teacher gives her a passing grade after that fiasco. Also, Nikki tries to ask Brandon, but he's on a field trip with the photography staff at a local newspaper community.
Tuesday, February 11
Nikki is not looking forward to shopping for Valentine's cards because Brianna wants to get the Princess Sugar Plum Valentines. However, every store they go to is completely sold out of them. Worse yet, the clerks are using earplugs to block out the sound of all the little girls who are too late and cry about it. When they go to the grocery store, Brianna happens to find the last of the PSP Valentines in stock and gets them, but not before falling from a height that forces Nikki to save her. At least Brianna got the valentines.
Wednesday, February 12
Brandon is back from his field trip and MacKenzie is busy putting up posters asking people to vote for her for Sweetheart Princess. Nikki tries to ask Brandon to the dance in biology class, but she gets distracted by the pop quiz announcement. She has to diagram the Krebs Cycle and ultimately flunks, causing her teacher, Ms. Kincaid, to talk to her. If she keeps up with her failures and goofs off, she will have to send a note home to her parents.
Thursday, February 13
Nikki decides to make a Valentine card for Brandon that has an invitation to the dance on it. When she tries to give it to him before class starts, MacKenzie butts in on them. Class starts and she decides to go ahead and give it to him while nobody's looking. But it slips onto the floor and Ms. Kincaid confiscates it and wants to see her after class. Brandon invites Nikki to go to the bakery across the street from Fuzzy Friends later after school, and she accepts. She talks to Ms. Kincaid and apologizes for what she did, and Ms. Kincaid sympathizes with her and gives her valentine back before warning her that she must not cause any shenanigans in class again. Just then, she gets a text from Brandon which was meant for MacKenzie. This shocks Nikki to the point where she checks the final item off the list, confirming any doubt she had about her relationship with Brandon. Heartbroken, she tears up the valentine she made and flushes it down the toilet.
Friday, February 14
Nikki has a nightmare about being her own Cinderella and meeting Brandon as a prince. When it strikes midnight, MacKenzie appears as a demonic monster. Nikki wakes up and finds that there was a blizzard and the schools are closed. She believes the dance was canceled and makes some fudge to enjoy. She and Brianna then give their parents valentines. She even gets a text from Brandon wishing her a happy Valentine's Day.
Saturday, February 15
Nikki is watching Oliver for a little while. Brianna doesn't want to play at first. Nikki tries to get her to play, but Brianna still refuses. She thinks Miss Penelope could be a better babysitter. Nikki challenges her and is going to see how it goes. While the kids play, Mr. Buttons suddenly disappears.
Sunday, February 16
While Brianna and Oliver look for Mr. Buttons, Nikki creates a new sock puppet named Maxine. This cheers Oliver up as they continue the search. Soon, they find Mr. Buttons behind a pillow. Oliver is so happy, he thinks Nikki is the best babysitter ever. He also gives Brianna a valentine, showing he enjoys being her friend. The two of them are very happy to be friends. Maxine is going to move into Nikki's sock drawer.
Monday, February 17
It's unofficially Valentine's Day at WCD and Nikki doesn't want to talk to Brandon about what had been going on. At gym, the teacher assigns swimming buddy exercises. She pairs Nikki as the drowning victim and MacKenzie as the savior. Unfortunately, while Nikki is well in-character, MacKenzie asks a bunch of hypothetical questions about the drill. Embarrassingly enough, she was drowning in only four feet of water. After the pathetic display that happened, the teacher gives them a written test. Everyone at first blames MacKenzie for not saving Nikki, but she blames Nikki for drowning in such shallow water and they all side with her.
Tuesday, February 18
To her horror, Nikki finds the picture that Brianna accidentally texted to her friends plastered in the hallway as a Sweetheart Princess campaign. She doesn't know who put them up there or why. She and her friends immediately take them down.
Wednesday, February 19
Today's swim skill is diving. Each diver must swim through seven rings. Nikki comes wearing scuba gear and, due to it, only gets a D.
Thursday, February 20
Brandon tries to talk to Nikki in the library about what had been going on. They said that one or the other had been too busy to hang out with the other considering what had been going on. They didn't know what had happened, but soon they settle on hanging out at Crazy Burger on Saturday. Zoey and Chloe saw the whole thing.
Friday, February 21
Nikki looks through her wardrobe to find the right outfit to wear at Crazy Burger tomorrow. She throws together a bunch of her clothes and eventually comes up with the perfect outfit.
Saturday, February 22
Today's the day Nikki hangs out with Brandon at Crazy Burger. When they arrive, they just casually wait for their food to arrive, bored to an extent. Soon enough, their food arrives and they talk about a few things. After a few nervous sputters, Nikki ultimately asks Brandon if he would go to the Sweetheart Dance with him. He accepts and Nikki is overjoyed.
Sunday, February 23
Now that Nikki is going to the dance with Brandon, she needs a dress to wear; none of the dresses she finds at the store really fit the criteria.
Monday, February 24
Nikki's mom offers her to wear her old bridesmaid outfit (which makes her look like a mermaid), but Nikki is a little uneasy. They schedule to shop for a dress on Wednesday, which is just two days from the dance.
Tuesday, February 25
Today in gym, the class is having timed swim races. Since Nikki can't swim very well, she fears she'll take too long to finish. When she swims, she discovers something in the water and believes it's a shark's fin, thinking a shark is in the pool. She swims for her life to the end and not only wins the race, but also sets a new school record for fastest swim time. Zoey and Chloe find out that it was just a scuba fin that scared Nikki. It was motivational enough to get her to swim superbly.
Wednesday, February 26
When Nikki and her mom go shopping for her dance dress, they find none left. The other girls who are going to dances and shopping for dresses are going at each other like some kind of war zone. It's hectic. Nikki is still reluctant to wear that dress her mom suggested.
Thursday, February 27
Brianna, AKA Miss Bri-Bri, approaches Nikki and wants to take her to an appointment. Nikki is hesitant at first due to what happened last time she let Brianna work with her. But she soon gives in and goes to see what it is that she wants to show her. They go to the bathroom where a package hanging in the shower for Nikki. There's a letter from her grandma that says that she got her a dress for the dance. Nikki opens it and finds a gorgeous dress she can wear to the dance. Miss Bri-Bri has worked her magic.
Friday, February 28
Tonight is the big Sweetheart Dance, and Nikki is more than eager to go. With her new dress on, she is ready to have the time of her life with Brandon. At that very moment, before she leaves, she gets a text from Brandon saying he came down with the flu. She is devastated that at the last minute, he canceled their plans. Heartbroken once again, she cries as Zoey and Chloe try to tear her up. She tries to get them to go without her, but they won't allow it and order her to come. She gives in and they go to the dance. While Nikki scarfs on some Wing-Dings, Zoey and Chloe tell Theo and Marcus about what happened with Brandon. But the guys tell them that Brandon is here, much to the girls' shock. Nikki sees for herself that Brandon is indeed at the dance...with MacKenzie. She comes to the conclusion that he lied to her and didn't want to go with her in the first place. She rushes to the girls' bathroom to cry in solitude. Zoey and Chloe try to cheer her up, but she just wants to go home. As they leave, they talk to Brandon about what had been going on and he wants to see her. Chloe was even planning to resort to physical violence. Her friends look around and find Brandon wanting to talk to Nikki about what's going on. Brandon is so determined to talk to Nikki about the situation that he resorts to going into the girls' room. As Zoey and Chloe leave the room, Brandon and Nikki discuss the circumstances that have been going on. As they argue, Nikki gets another text from Brandon, even though Brandon was with her the whole time and not using his phone once. She shows him the message and he tells her that he didn't send it because he wouldn't talk about boogers and snot, he never used a smiley emoji, and he lost his phone a month ago, around the same time as his party was ending. Someone else took his phone and has been the one sending these texts to Nikki. And Brandon was only with MacKenzie because she came up to him. So, it seems that all of this was a huge misunderstanding due to a nasty prank. They make up and try to find who could have taken Brandon's phone. Nikki decides to send a decoy text to it so that the person will read it and follow it. She says he got her a necklace and put it in dumpster #1573. Now they wait. They head back to the dance and enjoy some more wing-dings. MacKenzie comes up and notices Nikki there. She shows her one of the posters with Nikki's bad hair photo, one that she forgot, and it's revealed that Brandon was responsible for putting the poster up in the first place. Just when Nikki thought she could finally put it behind her, she loses her trust in Brandon and starts to break down again. And Marcus and Theo helped a little. Now, the night seems truly ruined. Soon enough, the Sweetheart Princess is about to be crowned. And the winner is...Nikki! She got the most votes because Brandon wrote a special campaign slogan on the back. Nikki wins the title of Sweetheart Princess and gets the tiara. Soon, every couple gets their picture taken. Meanwhile, MacKenzie is looking through the dumpster for the so-called necklace from the bait text. Turns out she was the one who had Brandon's phone all along. No surprise there. As the dance comes to a close, a slow song starts to play, and Nikki dances with Brandon, happy to have had a wonderful time.
- This is the second time a dance is featured in the series. The first time it was featured was on Tales From a Not-So-Popular Party Girl.
- Oliver makes his first, and so far only appearance in this book.
- Jessica does not physically appear in this book. However, she might of being either one of the CCPs Nikki mentions or in the background, making a background appearer.
- In this book, it seems that Brianna has lost her irratational fear of the tooth fairy. Instead, she now has an obsession with diva hair salon shows on TV, calling herself "Miss Bri-Bri, fashionista-hairstylist to the stars", constantly bugging Nikki about it. This will continue through the books, and Nikki needs to stop it, like she did with the tooth fairy in Tales From a Not-So-Popular Party Girl.
- Nikki reveals that Chloe Garcia and Marcy Simms begged her to take over her Miss-Know-it-all column for a month, with a special Crush Crisis Love Advice column, so she did. She almost sent a letter to them, asking why love is such a cruddy thing.
- In this book, Marcy's name is accidentally spelled as Marcie.