The theme is: "The Empires of the Future are the Empires of the Mind."
The official website is here, and it's simply festooned with information and teasers. Of note is that there are four stages for events, and that they are largely themed - which makes it easier to figure out which you're most interested in - however there is a Master Schedule if you just want to try to take it all in. Events go from Sunday, 22 June to Sunday, 29 June and then the sims remain open without events until Sunday, 6 July for a whole two weeks of celebration.
Edited to Add: For some reason the schedules for the entire event are down, but the schedule for day one Is Here, and presumably more days will be done in the future on the website.
The Main Stage is, as always, the Cake Stage (slurl), built in between two sims in order to allow for larger crowds. The events there vary widely, from music to dancers, and there are numerous places to watch the action from or dance, including flat platforms floating in the air, blocks hung along the walls, and two long rooms along the edges with seats. Or, if you feel like it, you can congregate on the floor and dance.
The Live Stage (slurl) offers up a wide variety of live acts in the palm of your hand... or someone's hand at least. It's on a single sim, so smaller crowds, and gives a really good perspective on the amazing variety of live acts going on in Second Life. If you want to get an overview of that community, hang out here and the acts change on the hour.
The DJ Stage (slurl) takes us into a World Dominating Mastermind's Mountainous Lair. It's on a single sim, so smaller crowds, and offers up the best disk jockeying Second Life has to offer. If you want to get an overview of that community, hang out here and the acts change on the hour.
The Auditorium (stage-side slurl / green-side slurl) is a smaller, double sided stage offering up a wide variety of events, from fashion shows, to plays, to lectures on Second Life specific topics. to dramatic arts. One one side is a standard proscenium stage with seats in front of it. The other is more of a platform in the round, offering up bench seating.
Second Life's official Eleventh Birthday celebration starts at 10am SLT/PST and the first events start at 12NOON sharp on all four stages.