I want to experience the Festival Cervantino in Guanajuato. Any suggestions about where to stay?
At this stage, I'd suggest you stay wherever you can find accommodation :S The city is literally packed to the brim during the festival, and all hotels, hostels, guesthouses and rooms are booked. A lot of people have to stay in the nearby towns.
The best weeks to go are the first or the last week of the festival, which is when it's usually not as crowded, but even then.
FWIW, my favorite place to stay in Guanajuato is Hostel Casa de Dante. The place is awesome, it's fairly close to downtown and the owners are great. The only downside is that it's in an alley and you have to climb about 150 steps to get to it. It's still worth it, imo. I stay there every time I'm in Guanajuato.
For the festival, maybe try couchsurfing as well, since you might be able to find something easier that way. Be careful and all that stuff, though, you ARE staying with a stranger.
Good luck! If you happen to find a place to stay, the festival IS a lot of fun :)