The double XP event will be available across all platforms, but the Xbox 360 has the added benefit of starting with a "Game with Developers" session. The event will run from 1PM EST on Friday through 1PM EST Monday, December 17th.
Along with the double XP, this weekend will also kick off Black Ops 2's clan competitions. Clans can sign up for one of three competitions on the Call of Duty Elite website, and earn double XP while playing. The three options include Core Domination Captures (Fri., 1PM EST), Core Kill Confirmed (Sat., 1PM EST on 360, 5PM EST on PS3), and Core Team Deathmatch (Sun., 5PM EST on 360, 4PM EST on PS3).
Treyarch promises it will, "soon be expanding the options over the coming weeks," meaning even more clan competitions are on the way. Unfortunately, mobile sign-ups for the competition are still being worked on, so the only way to make sure you take part is by signing up online.