From
MuggleNet | The ULTIMATE Harry Potter Fansite - Deathly Hallows, Order of the Phoenix, JK Rowling, rumors, news, forums, images, video, and more! 10 Years Since the Downfall of Lord Voldemort 
As revealed in ITV's JK Rowling Documentary
which aired back in December, May 2nd, 1998 was the day Harry finally defeated Lord Voldemort within Hogwarts School of Witchcraft and Wizardry.
A couple of visitors have asked us to post this wonderful anniversary. And why not? The Battle of Hogwarts within
Deathly Hallows was one of the coolest scenes ever created by Jo. It held what we've been waiting to witness since closing
Sorcerer's Stone. And who can forget Molly Weasley's infamous line to Bellatrix Lestrange: "NOT MY DAUGHTER, YOU BITCH!"?
So here's to Harry and his triumphant win against He-Who-Must-Not-Be-Named. Leave your thanks to Harry and Jo in the comments, and ask yourself: Do Wizards have a name for this anniversary?
LONG LIVE HARRY POTTER!