This year, the last day for the party was December 19, a mere two days before we arrived.
As for whether it is a crazy time to go – just make your reservatiosn for dining well in advance, and go to the parks early or stay late. We just got back from a late-December trip, and in my TR, you will see plenty of photos taken as late as 10 a.m. where the parks were almost empty! It didn’t get crowded until lunchtime – except for Toy Story Mania and Soarin’. Those two rides were packed from the minute the parks opened to the minute they closed.