The date is Halloween night. It is a block+ which gets decorated as places in the HP universe, mostly Hogwarts locations, for all the trick-or-treaters. They get upwards of 1500 trick-or-treaters these days, and since they block traffic for much of it, it is considered one of the safest, most fun places to go trick-or-treating in the area. Not to mention a great place to find a large appreciative audience to show off HP costuming.
Not a private block party
It is hard to see the crowds in any of these shots, but here are some photos of last year:
http://picasaweb.google.com/theagray/HarryPotterHalloween2006
http://www.annepaoli.com/ (Gallery 3)