I had a few ideas.
A) GladeClan celebrates a sort of feast, which is always the first day after Leafbare's first snow, when they celebrate all they were blessed with in Newleaf-leaffal and eat their last big meal until Newleaf. Kind of like Mardi Gras.
B) Non-canon adventuire. We have them celebrate Christmas then pretend it nevrr happened.
I personally prefer A. Feel free to suggest.
(PS: Like cats,