Everybody keeps complaining about offending people based on their real life religions. Guild Wars fixed this by creating a winter holiday called Wintersday that incorporated all of the secular representations of the Christmas season, ie presents, snow and snowball fights, candy canes etc. Bungie could do something similar and call it Light's Day in which the guardians celebrate living through another year fighting against the Darkness and decorate the tree in the tower with lights to represent the Light with which the guardians fight the encroaching Darkness. This is one of the wonderful parts about creating a world like Bungie has done. This way you can stil celebrate the holidays and not offend anyone by excluding their beliefs.