I thought the snow showing up in GTA V was a nice touch. There is going to be a lot of diverse players running around the city from differing backgrounds, so I wouldn't expect the city to be decked out in Christmas lights, if anything with all makes of humanity crammed into a small space you would see a focus on the event that wiped out humanity and the presence that saved it. So instead of Christmas trees I would imagine merchants selling mini travelers and other symbols so alien to our own perspective as say Muslims or Christians in life affirming celebrations in the Final City of Humanity that Bungie and a creative sci fi writer couldn't turn into an alternative events in the calendar.