North America:
Thursday, May 12, at 9:30pm (21:30)

Japan/Asia:
Friday, May 13, at 11:30

Europe:
France: Friday, May 13, at 19:00
Germany: Friday, May 13, at 19:00
Italy: Friday, May 13, at 23:30
Netherlands: Friday, May 13, at 19:30
Poland: Friday, May 13, at 21:00
Portugal: Friday, May 13, at 20:00
Romania: Friday, May 13, at 20:00
Scandinavia: Friday, May 13, at 22:00
Spain: Friday, May 13, at 21:30
UK: Friday, May 13, at 20:00

Australia:
Friday, May 13, at 19:00

More info can be found here

Lucky americans, getting to see the Xbox360 a day early :P