I once dated a girl whose birthday was October 4. This was a long, long time ago (pretty sure I picked her up for our first date with my horse-drawn carriage), and we've barely spoken since, but even with my crappy memory, I'll always remember her birthday.

Why? Because I forgot it once. And once was all it took.

Anyway, theme - I always have trouble with theme. My usual route is to come up with the story first, at least a sketchy outline of it, with maybe a few key scenes in my head. This leads to theme, rather than vice-versa.

Which is not to say that there's anything wrong with coming up with the theme first. Far from it - most of us write because we have something to say, and that something is the theme. You can think of it as a kind of mission statement for your story: This story will be about friendship, and the things that test it and bind it; this other story is about revenge and how it affects the people involved. Just to pick two from the list at the link from the Calendar. (Of course, sometimes the real fun is in combining themes like these.) The story's "mission" then colors everything you write.

Whatever approach you take, theme is important. Just don't be surprised if it changes over the course of Prep or even NaNo.