August 16th! According to
Basket Full of Kisses, which is an excellent and (so far) accurate fan site.
It looks like they're sticking with the Sunday night thing, which is great as they're starting a lot later than last season and need to be a bit careful about competing with network schedules come September. Mad Men is my favorite show on TV right now, (why, oh why couldn't 'The Wire' have gone on one more season?) so I'm super excited about having a start date for the new season. One thing I'm hoping is addressed in the new season (and it might not be, 1963 is a tad early for it), is the commercialization of the counter-culture movement.