Christianity and Christmas have lasted for about 2009 years now (historical say Jesus was born around 6 A.D), so why should 700 years change anything?
Seriously, sound better and find a smarter excuse
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Put some snow in the tower, give Xûr a Santa hat, give everyone a free legendary gun that doesn't affect 90% of the player base and slap on a name like "Travelmas" or "Festival of the Traveler".