I thought this XP boost was only running on July the 4th, but it will be kicking off today already, and presumably running until after the US Independence Day celebrations have finished.
It looks like it starts at around 3pm (GMT), according to GearsViking:
8.5 hrs till Gears 2 XP event starts – 4 x HOD Ammo, 4 x Mortar Ammo, Horde = Tickers, Boomers and Flaming Bloodmounts and 21x XP about 2 hours ago via web
@albi4 this weekend. 21x XP 12:37 AM Jun 30th via web in reply to albi4
