The hickory shad start running first... In most years, you can start catching a scattering of hicks around mid March. By early April they are usually there in full force. About that time the first American shad make a showing. Not many in the Rapp but there are a few. I live 15 minutes from the fall line on the Rapp so it is a short drive, but I don't think the Rapp has the best shad fishing...
I have been fishing the Potomac a lot more. I really worked it hard last year. I had several days where I caught over 100 hickory and American shad. I caught one American last year that was 6 lbs 3 oz. I wish I had my camera to get a shot of her! Here are a couple hickory shad I caught last year. Hicks are mini tarpon!


The area near Fletcher's Boathouse is great.
http://www.fletcherscove.com/fishing_report.htm Check the link, it will give you somewhat up to date fishing reports and you can check past years... That area of the Potomac is awesome! In one cast you could hook into a smallie, largemouth, striper, walleye, channel cat, blue cat, tiger musky, several species of sunfish, yellow perch, white perch and even a very lost hatchery rainbow trout! Last year I saw a snakehead get caught in that area for the fist time...
It is probably my favorite spot to warmwater freshwater fish because of the amazing variety of fish... If you want to make the trip, I'd be glad to take you out and show in the area.