I just decided to visit Ubeda and Sevilla (where the most famous Semana Santa processionals are held) during the week leading up to easter. Depending on where you'll be visiting you may have trouble finding a room. I emailed over twenty five hotels/hostals in Sevilla and finally found a room. Of course, prices are doubled during this period. Two weeks after easter in Sevilla is the April Fair and you'll run into the same issues. If you're planning on Sevilla during this time then you need to start looking now. I have the names of a couple of hotels that had availability but were a little outside of the Santa Cruz barrio. If you need them let me know and I'll get you their names and information. I think that the towns in Andalucia in particular will be harder to book. I know Granada is also difficult.

Hope this helps.