A christmas hat is an item in Evony TKR (The King's Return). It can be found after killing certain monsters or bosses, with some of them listed below.
The description of Christmas Hat in Evony TKR says:
Open to receive one or more items from it.
Christmas Hat is known to drop from a number of bosses or monsters upon killing, including Cranky Snowman.
Note: This list may be incomplete.