The Metro system doesn't open until 8am on Saturdays and Sundays. Yeah, it sucks, just ask any DC resident. If you plan to party until late at night in the District and if you need to take the Metro to your suburban hotel, plan ahead. Metro stations close shortly after midnight, so be sure to check your station's closing time. Cabs are plentiful in downtown DC and rates are OK if you aren't going far.
Fares can be very expensive for trips to suburban hotels.