Mukai, What exactly are you looking for? You can have a basic event where christmas mobs spawn randomly and give extra exp, or a massively complex event with killing certain mobs in a certain amount of time, then delivering an item to someone before the time limit runs out and getting back to collect your reward. It really does depend though on how much work you want to put into the scripts/events etc.
I'm not the best script writer but I'm pretty good with them and will gladly give you some advice on scripts.
Issy
P.S. Why a Christmas event this early in the year? There's Easter in 2 weeks, Summer Holidays to plan for, Halloween and bonfire night all to come before christmas