A Birthday, Not a Break

Something awful happens. The lovers finally break up, the heroes get captured, a beloved character dies, The Bad Guy Wins...and after the smoke clears, one of the characters suddenly remembers that today is their birthday.

Used to heighten the dramatic irony of something so tragic happening on what is supposed to be a happy day, and symbolizing how the character(s) will never be the same.

Compare/Contrast Ironic Birthday, which is when the bad thing in question has to be kept a secret, and the character in question doesn't remember his/her own birthday until the party.

Examples of A Birthday, Not a Break include:

Anime and Manga

The events of Dragon Ball Z: Battle of Gods occur during Bulma's birthday party.

Comic Books

In Ultimate Fantastic Four, Dr. Doom, age 11, had his father drop his entire family history on him, and then leaves him alone in the parlor. Several hours later, Victor realizes that it's his birthday.
In Trese, it is only after a major battle with an old war demon that the heroine, Alexandra Trese, and her companions remember that it is her birthday. By this time, a lot of damage has been done to the city, Trese herself is recovering from some battle wounds, and her two sidekicks have very nearly been murdered by their own father.

Fan Works

The My Little Pony: Friendship Is Magic Fanfic Pinkie Pie, and Nothing More explains her behavior in "Party of One" this way. Basically, her parents decided to send her away from the farm to pursue her dreams, without giving her any choice in the matter. When she asks why they sprung this on her birthday, she receives a blank stare and realizes they forgot.
In the surprisingly good Young Ones fanfic The Unforgiven, this trope gradually unfolds through Miranda Balowski’s journals and memorial service: she spent her sixteenth birthday giving birth to Vyvyan, and her parents threw them both out of the house shortly afterward.

Film

In The Room Johnny discovers his future wife and best friend had an affair, his future wife was leaving him and his best friend no longer likes him. He ends up killing himself because of it. And this all happened on his birthday.
In Amityville 2: The Possession Sonny succumbs to Demonic Possession and slaughters his parents and siblings shortly after his birthday party.
In House on Haunted Hill, Evelyn Stockard-Price's body is rotted out while her soul is sucked out by the Darkness on her birthday.
The first victim of the demonic invasion in Demons 2 was a girl named Sally Day who was celebrating her birthday.

Literature

In Tamora Pierce's Protector of the Small series, Kel is out training to become a knight, and rather focused on not freaking out about climbing tall trees, and covering her tracks. She then realizes its her birthday.
In A Series of Unfortunate Events, when the Baudelaires are in jail for allegedly murdering Jacques in The Vile Village, Klaus suddenly remembers that it's his thirteenth birthday.
- Violet goes through the same thing in The Grim Grotto when they return to the abandoned submarine.
In The Great Gatsby, after Tom, Jay, and Daisy have their big confrontation, Nick Carraway recalls that he just turned thirty today.
Percy Jackson & the Olympians: The Last Olympian: Percy only realizes it's his Dangerous Sixteenth Birthday AFTER he realizes the Great Prophecy had been fulfilled (ending the war).
Enola Holmes: Enola is horrified by the day her mother chose to ran off on. Slightly subverted, in that her mother had her reasons . . .
PK Pinkerton in The Western Mysteries comes home to find his parents scalped, his father dead and his mother dying. He doesn't remember it's his birthday until he sees his favorite kind of cake on the table. Red licorice strings spell out "HAPPY 12th BIRTHDAY PINKY".

Live-Action TV

In the Twilight Zone episode "The Shelter", a suburban doctor's birthday party is interrupted by a nuclear alert, followed by all the guests desperately trying to break into his fallout shelter, which has only enough room for the doctor's family. The alert turns out to be a false alarm, but the relationships between the doctor and his friends are damaged, perhaps irreparably.
Buffy the Vampire Slayer: Buffy's birthday in seasons 2, 3, 4, 5, and 6 is marked by some sort of tragedy or crisis that she has to deal with.
A minor example, in an episode of Friends, Phoebe has to break the news to Precious, the woman whom Mike was in a relationship with while he and Phoebe were separated, that not only are they getting back together (thus ending his relationship with Precious) but he also proposed to her. Precious then remarks "This is the worst birthday ever!"
One episode of Walker, Texas Ranger has the daughter of a deceased cop Walker was friends with raped when she goes to her very first bar on her 21st birthday.

Theatre

In Julius Caesar, Cassius realizes it's his birthday right before the battle where he's killed, after having noticed a ton of bad omens about the battle.

Video Games

In Ace Combat 04: Shattered Skies, the AWACS Sky Eye tells the pilots it's his birthday in the Back From the Brink mission, implicitly asking them to give him victory for a present. Happens again exactly one In-Universe year later, during the decisive battle for Farbanti.
In Ace Combat 5: The Unsung War, Grimm reveals that his birthday is coming up next week during a quiet break between the two final battles.

Western Animation

The Simpsons:

Homer: I can't believe it. I'm being mocked. By my own children. On my birthday!
Bart: It's your birthday?
Homer: Yes! Remember, it's the same day as the dog's.
Lisa: Santa's Little Helper, it's your birthday? Ooh! We've gotta get you a present.

Real Life

Former Buffalo Bills Quarterback Jim Kelly shared a birthday with his son, Hunter. On the day that Hunter died, it dawned on his grandmother that he died on his best friend's birthday.