First of All,Because Today's a special date!
Harry's birthdate,ihope you didn't forgot it as the Dursleys?
What else happened today!?
Harry Potter is born [PS/SS3].
Harry discovers he is a wizard, meets Hagrid, and visits Diagon Alley for the first time [PS/SS4].
Harry meets Dobby for the first time whilst the Dursleys have a jantar party, and the Elf uses a Hover Charm on Aunt Petunia's pudding, causing a serious disruption at dinner, and Harry to receive an official warning from the Ministry about using underage magic [COS2].
Aunt Marge arrives at the Dursleys' [POA2].
The guests at Harry's birthday jantar discuss the death of Karkaroff, and the disappearances of Ollivander and Florian Fortescue [HBP6].
Ginny kisses Harry, but they are interrupted. Scrimgeour arrives at The Burrow, gives them the items from Dumbledore's will, and argues with Harry about the way to wage the war [DH6].