Once again, the source is Zap2it. There is something a bit off about this addition though. The episode actually shows up as the 13th in line. So far, they has been 100% accurate about everything. Maybe it's a typo?
Regardless, have a Synopsis/Air Date after the break!
Title: Hearth's Warming Eve
Synopsis: The six friends are in a play about the founding of Equestria for the Hearth's Warming Eve's holiday pageant.
Air Date: December 17th, 2011