Tag them with a genre of Christmas, or something to your liking and then set up a filter for "no Christmas."
The Wiki should have information about filters.
That's exactly what I do, except I have a filter for Christmas and my playlists use a ruler to only include ticked tracks. I have a variety of unticked tracks that I exclude for other reasons, but when I want to include Christmas I go to the filter and tick them all (select all, right click, tick selected) and when I don't want to include them I untick them all.
If you're only playing your whole library, phred's suggestion is probably simpler.