We're not saying you have to reenact the whole Harry-Snape imbroglio at the big, pre-movie, 10-hour-long Harry Potter party at the Howard Hughes Center; nor do you have to cast any googly-eyed love charms. Of course, you can do both, but the main thing to do if you're a Potterite is to don that cape, put on those glasses, draw in that lightening bolt just so, and make for this wizard-y spectacular.
How wizard-y, and how spectacular, are two questions. The wizard-y bit: there are "wizard rock concerts" (the band name The Hermione Crookshanks Experience makes us smile) and and various games and contests and party-type stuff. The spectacular bit: Obviously the length -- party starts at 2PM and goes up to midnight, when "Harry Potter and the Half-Blood Prince" is released.
Whimsic Alley, that Potter-packed store in Santa Monica, is naturally behind the magical mayhem.