In celebration of the 4th of July in the states and Canada Day in um, Canada, from July 1st to July 6th all game modes in Gears of War 2 will net you 8 times the amount of normal XP. 6x XP during E3 was a big success, so Epic figured why not make it bigger and better this time?
Horde mode also receives a special tweak for this event with waves 1-9 being composed solely of Tickers and wave 10 being only Boomers. So if you've been thinking about firing the game up again, or have been feeling XP-starved, this week might be a good time to jump back in!
Source: Trueachievements.com