April/May is too early for the big events in the old time festival season.

The Mount Airy Fiddlers Convention (NC) isn't until June and Clifftop (WV) and Galax (VA) aren't until August.

If there is another Melvin Wine Birthday Party, it will be in Sutton WV, in April. Probably 10th or 17th.
http://www.augustaheritage.com/about.html#Augusta Year-Round

The Vandalia Gathering, Charleston WV, May 21-23, is a great festival in a beautiful venue that focuses on WV music.
http://www.wvculture.org/vandalia/

I've heard good things about the Florda State Folk Festival in May, but I don't know how much old time music content it has.
http://www.dep.state.fl.us/parks/folkfest/index.asp