Right now - this very instant - you can get 25x Experience Points in Gears of War 2, including Horde Mode. Epic Games promised a 1x multiplier for every thousand Facebook fans, and they're delivering. This trumps July's 20x XP weekend and lays track for the Gears of War 3 hype train - it's a good time to be a Gears fan.
There's no official word on how long the 25x XP lasts; since this is Labor Day weekend, it's likely good through Monday. But, really...why would you wait that long anyway?
Update: according to tipster Wildcard Zen, the event is still going until midnight Pacific Time on Monday night.
Source: Gearsofwar.xbox.com