* The Dursleys' Neglect: The Dursleys were already terrible to Harry, but on his birthday, they actively ignored him, even refusing to acknowledge the occasion. This reinforces their cruelty and Harry's loneliness.
* His Aunt's Cruel "Gift": Petunia gave Harry a single, wrapped-in-newspaper, withered-looking plant. This was meant as a mocking gesture, highlighting their disdain for Harry's magical abilities.
* The Arrival of Dobby: While initially seeming helpful, Dobby's interference and attempts to keep Harry away from Hogwarts caused a lot of trouble and ultimately led to Harry getting injured.
* His "Prison" in the Cupboard: Harry was trapped in the cupboard under the stairs and forced to watch a distressing news report about the attacks in Hogwarts.
* The Threat of Danger: Harry knew something terrible was happening at Hogwarts, and he was unable to help. This made him feel powerless and anxious.
However, it's important to note that other birthdays in the series might be argued as equally bad, or even worse, depending on your perspective.
For instance:
* Harry's 11th birthday in *Philosopher's Stone*: Although he received his Hogwarts letter, he was still trapped with the Dursleys, faced with their hostility, and ultimately ran away to Diagon Alley to escape them.
* Harry's 16th birthday in *Order of the Phoenix*: This birthday was marred by the Dementor attack and Dumbledore's absence, leaving him feeling isolated and unsupported.
Ultimately, each Harry Potter birthday is different and has its own unique blend of terrible moments. While his 12th birthday was truly unpleasant, it's subjective to call it the "worst" compared to other traumatic experiences throughout his life.