I hope no one has done this.

Do they celebrate anything in Dragon City like Christmas, Birthdays, April Fools, Valentine's Day, Easter etc?
Or are they all boring and "Bah humbug!"?

No holidays so far. There's was Cain's award party in "Cain's Mutiny" of course. So they have parties, but holidays are unseen as of yet.

But of course, the writers won't dare show anything that looks like a religious holiday. Though there could be holidays that commemorate past historical events, like maybe founding of Dragon City, or the "Be Kind to Dragons Day" where all the humans walk for a change. :D

Oh no, I just hope that theres not a Dragon Booster Day because that Arther would never be able to fit his head in his helmet again:-)