Mid-May in SEPA should be approaching prime "sulphur" time... be sure to have a good selection of PT nymphs, emergers, rusty spinners, and dry's in sizes 14-18 (although the 18's can actually wait until the end of May when the dorothea's
are the dominant "sulphur").
Hatch times for the early (mid-May) sulphur's should be mid/late afternoons... pushing towards evening if you happen catch a hot spell. The Ephemerella
nymphs will be very active and exposed during the time you'll be there.
If you'll be near the Lehigh Valley I could point you to specific streams/areas.
A quick reference for the Sulphur Hatch: http://www.paflyfish.com/smartsectio...-are-here.html