Hi you plan to go on saturday so I'll give you the exact way to do this:
1. Go to the MetroRail (subway/elevated) station in Rockville. ROCKVILLE is on the Metro RedLine. Take the Line that is going to GLENMONT, DO NOT take the train to Shady Grove.
2. Get off at the FORT TROTTEN STATION, this is a transfer point to the GREENLINE on the MetroRail. Take the train to GREENBELT.
3. When you get to the Greenbelt station, get out of the station and look for MetroBUS route B30 (there are metrobus signs with different route numbers, look for route B30). This bus will take you all the way up to the Baltimore Washington International Airport (B.W.I.)
4. When you get to BWI find out where the Light Rail station is, take the Light Rail to Pratt St. Convention Center.
AND YOU'RE THERE!
This is a tried and true route.
The DC system map can be found at www.wmata.com
The bus is $2.50 from Greenbelt to BWI
The Subway in dc will be about $3.00
The Baltimore light rail is about $3.00 as well.
ALSO: DC Metro and Baltimore light rail is NOT 24hours.
If you plan to leave Otakon back home on saturday, leave around 9:00 and do the opposite of how you got there.
I would suggest going with a friend, so you're not alone in travelling at night just for safety and all. Also a friend may help with directions LOL.