Twice each year Monarch butterflies in North America make a long migration. The Monarch migration actually takes more than one generation of butterfly to complete. The northward spring migration begins in March. We are going to concentrate on the Fall migration southward. With fewer storms in the US in the fall and crisp morning temperatures, the Fall Monarch butterfly migration offers plenty of photo opportunities.
Beginning at the end of August, the Monarch butterflies begin funneling southward through the United States towards their wintering grounds in Mexico. There is a fairly consistent date range for the peak Monarch migration in each latitude. The map illustration will give you a range of dates when the Monarch butterflies should be plentiful in your latitude. In general, anytime between the end of August and the first of November will be a good time to look. However, concentrating your efforts during the peak migration for your area will yield better results and likely more large groups of Monarchs in one place.
Monarchs lay eggs on the milkweed plant so these plants are a good indicator Monarchs should be nearby. Adult Monarchs feed a variety of flowers but looking for lilac, coneflower, zinna, and the ever popular butterfly bush to find concentrations of Monarchs. It has been my personal observation that Monarchs seem particularly fond of purple flowers. Many local gardening and conservation groups plant Monarch "way stations" as well so that migrating Monarchs will have plenty of food.
Once you've found the Monarchs be sure to follow my tips on How to Photograph Butterflies to get great Monarch butterfly photos.