[spoiler]Firstly I hope that each and everyone of you had a great Christmas, both in-game and in real life.[/spoiler]
The people of the Last City have been through a lot of disasters, wars, and the time before the City was hell on earth. Naturally they would still celebrate holidays even after the end of the world, but it's interesting to note how Armageddon has influenced and changed the holidays we know and love.
Christmas is a time for joy, peace, and giving; it is arguably the most important holiday on earth. So, naturally, Eva Levante brought a new and changed version to the Tower. Dawning is a time to celebrate giving and prosperity, yes. But Rahool also tells us that the Lanterns of the holiday are meant to draw the light back to us in dark times, something that certainly reflects the apocalyptic world they live in.
[spoiler]As for peace being a part of the season....yeah not so much. Instead we kill our foes and use their bodies to make cookies...[/spoiler]
I think it's also interesting to note that The New Year celebration and winter solstice (especially that last one) have been absorbed into one mega holiday. This is due to the astounding diversity among the City's public. Perhaps, though I have no lore reason to believe this, some people still follow the old ways and celebrate Christmas along with the Dawning.