As I recall, no MARC service on weekends to/from BWI.

Another option depending on the time of day is that DC metro runs a bus from BWI to the Greenbelt station. Best part is that the fare is only $2.00 (then you need a Metro fare from there...):

http://www.bwiairport.com/4grndtrn/in_mb.shtml

Last bus daily is 10:50 PM

Edited to add: Here's the timetable:
http://wmata.com/timetables/md/B30.pdf

[This message has been edited by pdhenry (edited 02-10-2003).]