Dreaming about Christmas (especially if it's near Christmas time) can mean that Christmas is on your mind, or you have certain expectations regarding Christmas time.
Dreaming about Christmas when it's not that time of the year can mean you're remembering something that happened around Christmas, or you're feeling the "good cheer" or other feelings you usually associate with that holiday.
Dreaming about Christmases past can mean there's something for you to gain by reviewing your memories of those times.