Anybody know how to start Thomas' quest? I hear you're supposed to do it on the days of the dead, but couldn't get it started.

Your approval might need to be at a certain level before Thomas will talk about his past and his quest. Try to always speak with him in the library and in Darla's wing in the evenings. On the first Night of the Dead he should be in Darla wing and I think that's the first chance he can speak about his quest. I have always had Shawn offer to accompany Thomas into the catacombs, not sure what happens if we just say good luck.