We did this on Dec. 23rd last year. It's a 30 minute "sleigh" ride that leaves from the front of Wilderness Lodge. Basically, it's a carriage (obviously, cause there's no snow in WDW.) that looks like a sleigh with wheels. There are bells on the sleigh so it sounds very festive. They take you around the side of the lodge, out into the forest area. It was very nice and it helped my family get in the "Xmas" mood at WDW. We did it at sunset so that added to the feeling. I know that Dec. 24th sold out really fast.
I made my PS - reservation ahead of time because they were selling out. I think I did it at something like 60 days. But I can't remember now.