Beautiful place...it takes 3 hours from Morelia to reach this place followed by 1.5 hours on foot uphill (and 1 hour downhill) to see the butterfly colonies. The route is Morelia-Maravatio-Aporo-Ocampo-El Rosario. There is another sanctuary close by called  "Sierra Chincua", but locals say this one is more beautiful. Preferred months are Dec-Feb. Best to be there at the entrance of the sanctuary at 9:00 AM so you can see butterflies still in trees and some flying. If you are carrying kids below 3 yrs make sure you have enormous energy to carry them up and down hill. You can have some good Mexican food when you come back from the hill. From El Rosario you can go to Los Azufres (it takes to 1 hour approx to reach) to enjoy thermal water baths. Location is awesome but very tiring journey all together. Don't plan anything difficult for the next day.

For some information about the sanctuary and about tours from Morelia or San Miguel de Allende, go here.