MTA released its new weekend MARC schedules, starting Dec 7th:

The train on the Penn Line will make nine round trips on Saturdays and six on Sundays. The MTA says tickets will cost the same on weekends as they do on weekdays $7 between Baltimore and Washington; $6 between BWI Airport and Washington; and $5 between Odenton and Washington. Weekly and monthly passes will be honored.

Southbound trains from Penn Station will depart on Saturdays between 7:35 a.m. and 9:15 p.m., and on Sundays between 9:15 a.m. and 5:30 p.m.

Northbound trains from Union Station will depart between 9:02 a.m. and 10:35 p.m. on Saturdays, and between 10:40 a.m. and 7 p.m. on Sundays.
This should not be confused with the door-to-door train service offered for other stadiums such as Foxboro or Meadowlands. One would have to change to Light Rail or a cab from Penn Station to Camden Yards area. Unfortunately, there is no Sunday service on the B&O line which pulls into the Camden Station by OPCY.