OOOOHHH! I've been to some good itunes gigs there in previous years (Primal Scream, Vampire Weekend, Elbow, Paul Simon, Placebo).
The iTunes Festival used to be every day for a whole month but this year it has been renamed the Apple Music Festival and it's just ten gigs. They already announced four of them (One Direction, Disclosure, Pharrell, Florence & The Machine), but not the rest. Will keep an eye out!
How it works: Once you are logged in to itunes, you need to go to the itunes store and look for the 'festival' banner at the top. Click on the banner to win a pair of tickets (no more). You cannot buy tickets.
You can't really buy tix on the black market either, as you have to bring photo ID that matches the "winning" name for the tickets.
DON'T. LOOK. DOWN.