From email:

“EatWell DC invites you to come enjoy a third day of brunch on Monday, October 8, 2012. Commissary and The Heights will offer their brunch menu from 9am-4pm, Logan Tavern from 11am until 4pm, and The Pig from 11am until 3pm.”

and

Ping Pong Dim Sum
Chinatown, Dupont Circle
Time: 11:30 a.m. – 4:00 p.m.
Cost: $25.00 unlimited Dim Sum; $15.00 bottomless tableside mimosa bar

Matchbox
Capitol Hill, Chinatown, Rockville, MD
Time: 10:00 a.m. – 3:00 p.m.

Cafe Deluxe
Cleveland Park, Bethesda, Tysons Corner, Gaithersburg
Time: All Day

And don’t forget – you can drink late on Sunday night. From the Washington Post:

The D.C. Alcoholic Beverage Regulation Administration has granted 42 bars and restaurants permission to remain open round-the-clock on Monday, Columbus Day, and serve alcohol until 4 a.m., the first test of a new city policy permitting extended alcohol sales before holidays.

You can see the full list of bars that will be serving late here.