Meh, not sold on this idea. Destiny takes place how far in the future? Do you really think, after the horrific war that pushed us back to a single city on Earth that any of our current holiday traditions would still be around? Doubtful. On top of that, it'd need to include holidays from around the world, not just the ones celebrated in the US, as The City is all that's left of EVERYONE on Earth. That said, they might have some days they celebrate somehow, like when The Traveler beat back the darkness, but it'd likely be a somber event. I think it'd take some serious creativity and more imagination that I'm currently having to come up with some realistic holiday events in game (likely coinciding with RL holidays for convenience) that didn't feel cheesy.