Typically Sevilla, but plan on sleeping outside or crashing at a friend's house-- hostels and hotels are almost guaranteed to be overbooked just about anywhere in Andalucia during Semana Santa. It's a big deal. (I know this to be true, having accidentally landed in Spain in the middle of this and having a DEVIL of a time finding a place to stay in any town in Andalucia. Actually, we didn't-- we left.
) Oh, and expect to share the festivities with lots and lots of french and german tourists!
Semana Santa is typically Europe's school holidays. And Europeans LOVE to vacation in Spain.
Welcome to the boards!!!!!