Theme Writing

How many of you write in a "theme"? Like right now, there are tons of holiday themed stories out there. But you have to be writing them months or years in advance. I haven't written much of it myself, though I do have a few short stories that would fit into this category. But I've had a lot of questions about it I'd like to have some input on.

If you do write them, what do you do to get in "the mood?" How do you prepare for and research info to write about a "theme"? Do you feel like it's a hard place to "break into", because so much has been said about it so many times? How do you make it fresh and original?

Share, people!