I've already teased that Castle will be doing their very first X-mas themed episode this year, airing December 3rd. "Secret Santa" centers around a bearded man that literally falls from the sky in Central Park. The man is in full Santa attire, wearing "Kris Kringle" name tag. Over the course of the investigation, Beckett and Castle learn that Mr. Kringle was more naughty than nice. However, murder doesn't stop Rick and Kate from sharing their first Christmas together.
Source: TV Line