On Friday July 22 from 16:00 - 19:00 Eastern Time (4:00 PM - 7:00 PM), Rockstar is hosting a Social Club Multiplayer Event. Please see my calendar blog post for a full Calendar of known events with a time zone converter and a description of the Social Club events.
Rockstar has announced that Triple XP will be extended for several hours beyond the event, "staying on long enough early Saturday for all those around the world to get a piece of the action."
Although the official event is being held for PS3 only, both platforms get Triple XP. The next event for XBL will be Thursday, August 4 from 17:00 - 20:00 Eastern.
Even if you can't participate in the event, you can stop by the Social Club Multiplayer Events page to enter the contest for 20 Rockstar prize packs.