Each year, thousands of butterflies migrate from the United States and Canada to live among the the oyamel fir trees for the winter months.
By thousands of butterflies, I mean thousands!!! During early stages of the migration, police in the area of the Monarch Butterfly Biosphere Reserve in Cerro Pierto, Mexico, impose a lower speed limit so that the butterflies are less likely to be injured by motorists.
This reserve covers 200 square miles, and was proclaimed a UNESCO site in 2008.
Each year, from November to March, the park is open to the public, and people come from around the world to see the wintering butterflies: and there are a LOT of butterflies to see!
When should you go? Well, the population of butterflies peaks during January and February, and the park is open daily from 9am to 6pm from mid-November through March. Avoid weekends, though, or it'll be really crowded.
Additionally, there is a yearly Festival Cultural de la Mariposa Monarca, that takes place at the end of February. Hotels in the nearest town, Angangueo, may fill up, so plan in advance. For more info about traveling to the Reserve, check out this card (http://www.vingle.net/posts/302588-How-to-Reach-the-Monarch-Butterfly-Biosphere-Reserve-Mexico)