It is the Mickey's Very Merry Christmas Party. It is held in the MK on certain dates the end of November until about mid December. You need a separate ticket for the event, it snows on Main Street, they have Christmas shows through out the park, there is a special Christmas Parade, which is great. Some of the attractions are open, but not all. There is hot chocolate and cookies and you get a free photo.
When you book your tickets for the MVMCP, if there is availability at the Liberty Tavern you can make your reservations then, rather than waiting for 90 days before your trip.