Treyarch has announced (via PS3 Message of the Day) that there will be a Double XP weekend for Black Ops 2 starting at 10am PDT on Friday, July 26th and lasting till Monday, July 29th at 10am PDT for all platforms.

This is in preparation of the Vengeance DLC set to be released August 1st on PS3/PC .

SOURCE: Treyarch (Thanks @moriarty1975) 

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