Most likely Halloween as they could incorporate The Nightmare before Christmas into that. They’ve not got the talent to write a musical episode (unless they just plan on the actors singing the Disney songs). Christmas wise we’re probably looking at a Christmas Carol. Muppets did it and they come under the Disney umbrella (you used to get muppets clips on the Disney VHS tapes).
Christmas Carol is also a very classic piece of literature, and one that could fit with the Land of Untold Stories. Now I can’t help but imagine Rumple in the Ebenezer Scrooge role. lol!