December is the only month the whole world has holidays at the same time. I always take care i have a place to stay cause hotels triple also.
But also depend, flying to Mexico City from Europe is 3 times more expensive than go to Cancun and than Mexico City. I always use skyscanner and select first "whole month" to see what day is cheapest. If there is a nice stop in between flights i might stay some weeks. But i am a flexible slow traveller.
https://www.skyscanner.com