"Great news for MARC passengers!  Starting on December 7, 2013, weekend service will start on the Penn Line. Complete schedules are available below. The fares will be the same as weekday service ($7 Baltimore-Washington, $6 BWI-Washington, $5 Odenton-Washington) and weekly and monthly passes will be honored.

FAQ's: Here are some frequently asked questions about the new MARC Penn Line weekend service.

How many trips will operate on the weekend?
There will be 9 round trips on Saturday and 6 on Sunday.

Will there be weekend service on the Camden Line?
No. Camden Line passengers are welcome to use the new Penn Line service.
 One-way, weekly and monthly tickets on the Camden Line are always
honored on the Penn Line.

Will there be weekend service on the Brunswick Line?
No. Brunswick Line passengers are reminded of the MTA Commuter Bus 201,
which provides 7-day a week, hourly service between Gaithersburg Park
and Ride, Arundle Mills Mall, and BWI Marshall Airport and Rail Station.

Can I use my MARC weekly or monthly pass that I use Monday through Friday for the new weekend service?
Yes! You will need to purchase single ride tickets for anyone in your party
that does not have a MARC pass. Weekly tickets are good for unlimited
use beginning on the Saturday of the designated week until midnight of
the following Friday.

Will select Amtrak trains still honor MARC weekly and monthly tickets on the weekend?
No. MARC tickets will no longer be honored on Amtrak weekend trains.  Funds
used to pay Amtrak for this priviledge have been shifted to pay to
operate the weekend service. 

Will parking still be free at MARC stations?
Of course! All MARC Penn Line stations, except Penn Station and BWI, feature free parking."

Source: http://mta.maryland.gov/marc-begins-weekend-service-december-2013