So you happen to have a group of friends who think it's a special occasion, such as your birthday. What do they do, you say? They decide to throw a party for you in the comfort of your own home. When you come home, they all surprise you and celebrate. Wait. Why are you not happy?! We worked hard to make this party for you!
Oh, it's not actually a special occasion? Whoops.
This is the trope where characters don't realize that it isn't a special occasion, such as a birthday, anniversary, or holiday. This can be played in many ways. Either they forgot the date, or it's because they're just being too kind to the character. This can often be geared towards a grumpy character whom the others want to cheer up.
For birthdays, this is the inverse of Forgotten Birthday. This can combine with Scary Surprise Party for extra comedy value. Related to Ironic Birthday, which is when the character doesn't want to celebrate despite the fact it's their birthday. Compare with Birthday Hater for when it is their birthday, but they don't want to celebrate it anyway.
- In Future GPX Cyber Formula SAGA, Randoll comes to Asuka's apartment to give gifts for her, thinking that it's her birthday. Asuka then tells him that her actual birthday is March 14th. Funnily enough, it's actually Randoll's birthday (his is on July 7th).
- In The Witch of the Everfree, since Pinkie Pie doesn't know when Sunset Shimmer's birthday actually is, she just takes guesses, leading her to throw Sunset a "birthday" party every month or so.
- This Young Avengers comic. Apparently, it's their rebirthday (or something).
- Sort of happens in Yu-Gi-Oh! The Abridged Series:
Dartz: We awe hewe to cewebwate an event so fantastic that the whole world will wemember for years to come!
Alister: Happy birthday, great and powerful—
Dartz: It's not my birthday, man.
Alister: Oh. Then why is there a birthday cake over there?
Zombie Boy: Oh, that's mine. It's my birthday, celebrating 15 years of being a zombie.
- Played with in Alice in Wonderland. Alice comes upon the Mad Hatter, the March Hare, and the Doormouse celebrating what she thinks is a birthday party, but they're celebrating their unbirthday - which is any day that isn't your birthday. Alice realizes it's her unbirthday too, and they include her in the celebration.
- In Turning Red, Mei and her friends celebrate having raised the money for the 4*Town concert thinking it's the next day only to find out a few seconds later it's not.
- In the Steven Spielberg film Always, Pete (Richard Dreyfuss) throws a big shindig for Dorinda (Holly Hunter)'s birthday... except it wasn't her birthday. He keeps saying he remembered it, she keeps saying yeah, but he remembered it wrong.
- Played for deep black comedy with the surprise birthday celebration for Spats Colombo at the convention of the Friends of Italian Opera in Some Like It Hot.
- This is a plot point in Declare: history thinks Kim Philby was born on January 1 but in the book, his real birthday is on December 31, which he hides from everyone because he is functionally invulnerable on all days except his birthday.
- Invoked in two of the Diary of a Wimpy Kid books, where a character deliberately lies that it's the birthday of another character in order to get free dessert from a restaurant. First Greg with Rowley, then Mrs. Heffley with Manny.
- Garrett, P.I.: In Whispering Nickel Idols, Mafia Princess Belinda throws a 60th birthday party for her father Chodo, even though it's not the right day or the right year for it. Everyone she invites already knows this, but come anyway because they realize the party is just a front for a major criminal summit.
- The Worst Thing About My Sister: When Martina, a.k.a. Marty, is invited to Alicia's birthday party against her will, her mother, a dressmaker, makes her such a fancy dress that the clown thinks she's the birthday girl (and Alicia can't correct him because she's in the bathroom). Marty doesn't correct him either because she wants to assist the clown with his magic show.
- In It's Always Sunny in Philadelphia, the gang plans a surprise birthday for Charlie to make him feel better. But it isn't his birthday. Frank just said that to trick them into planning a luau party for his own birthday.
- Just Shoot Me!: Jack wants to get Maya a present for her thirty-fifth birthday, and decides that, after years of neglecting her as a child, he should give her "the gift of me." This makes Maya angry, because her birthday is 7 months from now, and is upset that her own father wouldn't know. Still, Jack is sure that it's someone's birthday... and that's when they both realize that they have forgotten Maya's mother's birthday.
- In the Cold Open for Kenan & Kel, Kel tells everyone it's Kenan's birthday and gets the audience to yell "Surprise!" when he comes out. After getting over the mild shock, Kenan informs Kel that it's not his birthday and points out that the cake he got him says "Happy Bar Mitzvah Benjamin".
Kenan: You got the wrong cake and the wrong date!
- M*A*S*H: In order to brighten the mood of the camp BJ tells everyone it's Hawkeye's birthday and they arrange a Surprise Party for him. Hawkeye "accidentally" lets slip that the same day is also BJ's wedding anniversary. Ultimately Hawkeye has to go out on a medical emergency so BJ recruits some miscellaneous extra to say it's his birthday.
- Inverted in the Seinfeld episode, "The Outing", where George, Kramer, and Elaine each plan on getting Jerry something special for his birthday; Kramer is the first to give Jerry his present. Jerry insists it's not his birthday, although the others insist that it is, to which Jerry comments, "Maybe you're right, maybe it is my birthday..."
- In one episode of Sesame Street, Telly tells Rosita to guess whose birthday it is and she mistakenly believes it to be Oscar's, then Slimey's, then Carlo's. It turns out to be Telly's.
- Daniel gets one of these in the series finale of Stargate SG-1 courtesy of Vala. Played with in that Vala knew full well it wasn't Daniel's birthday, but the crew of the starship he was on did not.
- Zig-Zagged in I Love Lucy, during the "Europe" arc—to make up for being away from Little Ricky on his birthday, Lucy decides to throw a party for a local boy who claims that it's his birthday, too. The boy winds up bringing a whole group of other kids who all claim that it's their birthday; Lucy isn't fooled, but goes along with it. Eventually the first boy feels guilty and admits to lying, and tries to get everyone to give back their presents—one girl says that it really is her birthday, however, and thus Lucy declares that everyone can keep their gifts.
- "It's Not My Birthday" by They Might Be Giants
It's not my birthday, it's not today
It's not my birthday, so why do you lunge out at me?
- Mick Foley has a preference for skits related to this trope:
The Rock: The Rock's birthday is May 2nd, you stupid son of a bitch!
- In one part of the memorable "This Is Your Life" segment on Raw is War, as Mankind, he brought a clown to sing "happy birthday" to The Rock as well as bringing out a birthday cake for him. Because he was absolutely unimpressed, this is what he has to say to Mankind:
- In one pictures for sad children comic, a guy receives a surprise birthday party on the day before his birthday, then on his actual birthday, then on the day after, then on the day after that...
- The 3-2-1 Penguins! episode "Compassion Crashin'" starts off with the penguins throwing a surprise birthday party for Jason and Michelle, only for Jason to point out that their birthday was two months ago.
- Back at the Barnyard: In "Chez Pig", the animals start a restaurant and come up with a song to sing to people on their birthdays. Freddy begins singing this song to random people or shouting, "Birthday!" at them because he likes having the opportunity to sing the song/talk about birthdays, even if it's not their birthday, which causes one of the guys whose birthday it isn't to shout at Freddy.
- Dastardly and Muttley in Their Flying Machines: In "Happy Bird Day" (the final episode), the Squadron is under the impression it's the General's birthday. At the end, it turns out it's Dastardly's birthday.
- An episode of Fish Hooks had the entire class thinking it was Clamantha's birthday, even though it wasn't.
- On Foster's Home for Imaginary Friends, Bloo keeps throwing surprise birthday parties for Mac, then using them to embarrass him in front of everyone. Of course, to keep Mac from suspecting them, he never does it on Mac's actual birthday...except for that one year when he did. "So basically, I'm never safe!"
- On The Grim Adventures of Billy & Mandy Billy convinces Grim to help him throw a surprise party for Mandy. Grim actually ends up feeling good about preparing for the party. When the two surprise Mandy, she asks Grim if Billy told him that her birthday isn't for several months. Billy says that's what makes it a real "surprise party". Grim breaks down.
- The Hair Bear Bunch: The bears sneak back into the zoo after a night out in town and fake being asleep when the keepers arrive at their cave. Peevly asks Hair where he got the party hat on his head, to which Hair says that they were celebrating Peevly's birthday.
Peevly: My birthday isn't for six months!
Hair Bear: Nothing like getting a head start, sir.
- The KaBlam! second-season episode "E Pluribus Kablam" opened as a big 100th episode celebration gala, but as Henry and June are opening the star-studded special episode, a stagehand whispers to the two that it's only the 24th episode (in production order, as it was actually the 20th in airing order). Naturally, June blames Henry for it, but among realizing they are being watched they continue it like a regular episode...
Henry: Hang on tight as we kick off KaBlam's fan-tabulous *cough24thcough* episode extravaganza, here's Sniz and Fondue!
- In one Phineas and Ferb episode, Monogram and Carl surprise Perry with a birthday party when he enters his secret lair. Perry's silent stare tells them it's not his birthday. Becomes a Call-Back joke later when Perry arrives at Doofenshmirtz's place. Doof mentions he got an invitation to the party but didn't go because he suspected it was a trap.
- In the beginning of the SpongeBob SquarePants episode "Opposite Day", SpongeBob and Patrick are annoying Squidward by throwing a birthday party for him, which really isn't, as the quote from above states it. This is what prompts him to move out of Bikini Bottom, which apparently backfired on him thanks to SpongeBob and Patrick.
- One Couch Gag for The Simpsons has the cast coming in to celebrate 500 episodes. But as the music begins, Lisa does the maths and reveals that this is actually episode 499.
- Enforced in Star vs. the Forces of Evil: Star never celebrates her birthday on the actual date because that's also Stump Day. She thinks celebrating her instead of the Stump is a sacrilege that invites its wrath. And she's more or less right.
- Invoked in the Steven Universe episode "So Many Birthdays"—Steven finds out the Crystal Gems don't celebrate birthdays, being ageless, and decides to throw them each a party anyway. They find all of the birthday rituals bizarre or downright terrifying.
- For some reason, Jason Statham's birthday keeps getting referred to as the 12th of September, even though it's actually in July. He usually politely thanks people for the birthday wishes, but also points out it's not his birthday.