There was a bus service one recent season to Whitetail on weekends, but it didn't last beyond one year, not enough business.
Check here for one day trips by Ski Club of Wash:http://www.scwdc.org/calendar.asp
Where do you live (rhetorical question)? There are links here to a bunch of other ski clubs in the region. If you check them out many offer day trips too: http://www.skicouncil.com/tripsbydate.htm
Welcome to DCSki.com I would expect there to be posts here this winter for more folks looking to carpool to the slopes.