Journey North -- Past Season Maps
You need to select the species. I searched on March in 2007 nothing, then April for first Monarch Sightings. I found something in April 2007.
http://www.learner.org/cgi-bin/jnorth/j ... 1177465550
Richmond VA -- April 24
The chances are that you could see them around the end of April if your area is not too developed. I hope you have plenty of milkweed planted. The more you have the better chances of attracting the Monarch butterflies. Early nectar is also important. I have seen them nectaring on dandelion blossoms.