The Mourning Cloak, like the Eastern Comma, overwinters as an adult and is often the first butterfly seen in the spring.  I often find it flying in the woods on warm, sunny days in late March. 

Although this photo was taken at Nelson Lake Marsh, about a week later at Fermilab I also found Mourning Cloak caterpillars on Willow.  The caterpillars feed in colonies, so they are easy to spot.