Birthdays. A celebration of the individual and their continued existence on this Earth, a personal and special day for the birthday-person, a day all about them. Except that, with only 365-366 days in a year and billions of people on the planet, no birthday is unique to the individual. Everyone shares a birthday with someone... and occasionally, a character may just run into someone else who has the same birthday as them.
Results may vary.
This may be a cause for bonding, if the characters decide to celebrate together or, at the very least, realize they have one thing they can both relate to. This may also be a cause for a rivalry, or inflame a pre-existing rivalry, as both characters may want the spotlight, and compete to have their party be bigger and better than their enemy's. It may even mean both characters are special somehow, if the birthday they share is a Significant Birth Date.
Regardless of the result, though, the fact that these characters have the same birthday is important to the story, not just a one-off gag or a random coincidence. The characters will acknowledge it and actually care about the (usually) coincidental connection they share, even if it's because they hate each other, and can't stand having anything in common.
These characters, of course, should ideally be unrelated. Twins are inherently born on the same day, note so this trope concerns those who aren't twins... that is, unless the point is to foreshadow a Separated at Birth twist, with the characters (and often the audience) not knowing right away. This also means other familial-relationships are allowed, assuming the emphasis is on the special shared date just as in a non-related-character example. They also don't have to be literally born on the same day down to the year, so long as they share the same birthday each year they're both alive.
Truth in Television, obviously. Everyone shares their birthday with someone. An interesting thing to note is that, according to the birthday paradox, it only takes 23 people to have a 50% chance of having a pair with the same birthday. In fiction, the fact that this trope is usually averted makes it similar to The One Steve Limit- while common in real-life, it's weird for fictional characters to have the same name, just as it's uncommon for them to have the same birthday.
- In Hitoribocchi No OO Seikatsu:, Bocchi Hitori and Kai Yawara discovered they shared the same birthday at age 5. Kai was upset that Bocchi kept calling her "Yawara-San" and refused to let her leave until she called Kai by her first name. When Bocchi explained the next day that she didn't use Kai's name because she wanted Kai to stay with her, Kai promised to stay with her and that they would always celebrate their birthdays together. Kai was Bocchi's Only Friend from this point until the start of the series.
- Both Fujiwara and Ishigami from Kaguya-sama: Love Is War were both born on Hinamatsuri (March 3rd). Fujiwara immediately begins complaining upon hearing this because she doesn't want to have to share her birthday party.
- Letter Bee: Lag and Sylvette are both born on the Day of Flicker. Being born on the Day of Flicker caused Sylvette's legs to be non-functional from birth, while Lag has an eye missing. As one may guess, the Day of Flicker later turns out to have been the sign of something bigger going on.
- Full Metal Panic!: Though not mentioned until well into the series, all of the Whispered around the world were born on within the same three minute timespan (though local time frames mean some are on different calendar days).
- In Asterix, Asterix and Obelix have been friends since day one because they were born on the same day.
- In Unexpected Surprise, Emma and Gabriel first meet when the little girl leaves her mom's office to cheer someone up during her birthday, and runs into the boss grieving, because it's also the birthday of his beloved late wife.
- 16 Wishes: Abby and her next-door neighbor rival Krista share a birthday. This is torturous for Abby, who is stuck watching Krista get huge parties, expensive gifts, and even a lot more respect from the adults than she gets. The plot is that Abby wants to have the best Sweet Sixteen ever, and uses a bunch of her wishes to knock Krista down a peg at the same time. This then ties into the reason why Krista bullies Abby: Abby stole Krista's best friend, Jay, from her during a birthday party, and Krista hated her ever since.
- In Identity, the characters slowly begin to realize that something eerie is going on when they discover they all have the same birthday.
- Downplayed in Harry Potter. Neville Longbottom and Harry Potter were actually born a day apart - 30th and 31st of July 1980, respectively - but this turns out to be an incredibly Significant Birth Date since the prophecy that pushed Voldemort to kill Harry's parents (and thus kick-start the whole plot) specifies that "the one with the power to vanquish the Dark Lord will be born as the seventh month dies." Voldemort took this to mean Harry but it could have also very easily been Neville, as Dumbledore notes while explaining to Harry the nature of fulfilling prophecies.
- In The Lord of the Rings, Bilbo and Frodo share a birthday, September 22. The story opens with their shared birthday party (Bilbo's 111st, Frodo's 33rd). After Bilbo leaves the Shire to go adventuring again, Frodo continues to refer to the date as "our birthday" and keeps up the tradition of joint birthday parties.
- In Snow Flower and the Secret Fan, Snow Flower and Lily ending up being chosen as each other's laotong in part because they share the same birthday.
- In Switch, the second sequel to Savvy, Gypsy is very energetic and happy, but her grandmother, Pat, is a Grumpy Old Woman, and they share a birthday, so Gypsy worries that she will also be grumpy when she's elderly.
- House of Anubis: Joy and Nina were both born on July 7th, which is a Significant Birth Date. Joy was believed to be The Chosen One because of her birthday, but wasn't born at the exact right time; this lead to the reveal that Nina, who had the same birthday, was the real Chosen One, being born at 7:00 AM. The two of them were, incidentally, born 12-hours apart.
- In the Space Cases episode "It's My Birthday, Too, Yeah", Radu lies and claims that he shares a birthday with Bova, because his species don't actually have birthdays and he's Never Had a Birthday Party.
- Played with in the Growing Pains episode "Birth of a Seaver". Maggie goes into labor with Chrissy on Ben's birthday, leaving Ben upset about being ignored until he has a heart-to-heart talk with a dying elderly man in the hospital. But the labor takes so long that by the time Chrissy is finally born, it's past midnight, and Ben is relieved that he won't have to share his birthday with his new sister after all.
- Similar to the Growing Pains example above, Full House has Becky go into labor with Nicky and Alex at Michelle's birthday party. The episode ends with the entire Tanner family singing "Happy Birthday to You" to all three of them in Becky's hospital room.
- Power Rangers S.P.D. has a variation where Jack and Syd decide to become Birthday Buddies voluntarily: Jack had grown up on the streets, Never Had a Birthday Party, and doesn't even know when it is; which causes conflict with Syd when he quashes her birthday plans without understanding what the big deal is. When they reconcile, Syd offers to share her birthday with him and he adopts her birthdate as his own.
- In The West Wing episode "Noël," it only further messes with Josh's already traumatized mind to find out that he and the suicidal Air Force pilot, Robert Cano, share the same birthday.
- Emily Owens, M.D.: Emily's and Cassandra's birthdays happen to be on the very same day. They're young doctors who used to be classmates in high school, Cassandra being the gorgeous popular mean girl who enjoyed tormenting Emily. Now they're interns in the very same hospital and Cassandra still enjoys toying with the very nervous Emily. Emily doesn't really celebrate her day and doesn't want anyone to know, but Cassandra reveals it to everyone, enjoying everyone's attention and Emily's embarrassment. Emily's friend Will forces her to throw an impromptu birthday party in her flat. Emily is also talked into playing the cello for her guests and everyone ends up being impressed with her musical skills. Cassandra on the other hand looks like a sourpuss and is unhappy the party doesn't revolve around her.
- In the The Wubbulous World of Dr. Seuss episode, "The Birthday Moose", the Birthday Bird takes everyone sharing the same birthday to Katroo for a huge party. In this episode's case, it's Thidwick the Big-Hearted Moose, Morton the Elephant Bird, Fox in Socks, Mr. Knox, and Fiona Phish. When Thidwick finds out that the Birthday Bird is unable to make it this year (due to being frozen alive in a block of ice at an arctic climate), Thidwick tries to take him and his friends sharing the same birthday to Katroo himself, disguised as the Birthday Bird.
- It's possible for your birthday in Animal Crossing to fall on the same day as a regular villager's birthday. It gets acknowledged as something unique and exciting.
Peppy: I can't believe it's your birthday AND it's my birthday too! This is, like, the best day of the year! Two birthdays, two cakes, and double the presents!
- Elsword: Elsword and Aisha shares the same birthday, in December 27th.
- Fire Emblem: Three Houses: Whatever birthday you put in for Byleth, the mysterious girl in their visions shares as well. She comments on this and feels connected to them because of it. Actually, that birthday only belongs to Sothis. Byleth just osmosed it due to Sharing a Body. Though even once their real birthday is revealed, Byleth's friends still celebrate their birthday on the day you entered.
- The Lizzie Bennet Diaries: Lizzie Bennet is very close with her best friend Charlotte Lu, who was born on the very same day as she was. Lizzie jokes that they have been friends since fetuses because their mothers were friends who frequented the very same book club and went to labour at the same time. There's a rumour that Lizzie as a child behaved like a brat during Charlotte's sixth birthday party and wanted to open all of Charlotte's presents. During Q&A from fans, one person said they sensed lot of drama in that story, since it must have been Lizzie's birthday as well. Charlotte snarks that it's precisely it: Lizzie had her own presents to open.
- The Arthur episode "Arthur's Birthday", adapted from a book of the same name, deals with Arthur and his classmate Muffy having their birthday parties on the same day. Arthur and his friends aren't sure whose party to attend until they come up with a solution: they turn it into a surprise party for Muffy at Arthur's house.
- In Avatar: The Last Airbender, it is revealed that the previous Avatar, and the Fire Lord that started the war (Roku and Sozin) were actually born the same day, and show that they were very close friends, even celebrating their birthday together. This all contrast with their later relationship as bitter enemies in their old age.
- Ben 10: Cousins Ben and Gwen Tennyson share the same birthday, and since they are both 10 years old that means they were born on the same day. Since they are both spending the summer travelling with their grandfather Max, they get into an argument over whose birthday should be celebrated on the actual date of their birthday.
- Subverted in The Fairly OddParents episode "Boys in the Band", where Timmy is having a miserable Forgotten Birthday alone at home with Vicky while his parents are going to the Chip Skylark concert. Chip Skylark then winds up at Timmy's house, held hostage by Vicky, and it turns out to be his birthday, too. Timmy, having initially blamed Chip for ruining his birthday, starts to bond with him over the fact that they're both having a miserable birthday... except that Timmy's birthday was actually the day after, and it was he who forgot. Although a later special claimed the two did share a birthday, so maybe it was his parents that forgot.
- In The Simpsons, the episode "The Springfield Files" reveals Homer shares his birthday with the family dog, Santa's Little Helper. However due to the Alien Episode plot that Homer is trying to prove he had an encounter, no one believes his story and instead gives the birthday attention to the dog.