In Washington, summer means certain things. Muggy days everyone knows about. Here are a few other things everyone may know about:

CONCERTS ON THE CANAL--Starting this Sunday and continuing every other Sunday through September 5, 1:30 to 4:30. Always free and always on the C&O Canal in Georgetown (between 30th and Thomas Jefferson Streets NW, below M).

MILITARY BAND CONCERTS--The U.S. Air Force Band starts its summer concerts June 1. Continuing through the summer, concerts are Tuesdays at the U.S. Capitol West Terrace and Fridays at the Jefferson Memorial. Free at 8.

Starting June 1, the U.S. Army Band plays Tuesdays at the Jefferson Memorial and Fridays at the West Terrace of the Capitol. Shows are free at 8 through August 31.

The Navy Band plays Monday evenings at 8 at the West Terrace of the Capitol. Those shows go June 7 through August 30, and Thursday night shows begin June 3 at 8:30, continuing through August 26. (Call 433-2525.) FORT DUPONT CONCERTS--Beginning June 25, every Friday and Saturday night through August 28 there's a concert in the park at 8:30. Free, and the first one is Della Reese; opening act, the Blackbyrds. At Minnesota Avenue and Randall Circle SE.

SMITHSONIAN FOLK FESTIVAL--The 16th annual Festival of American Folklife is on the Mall at 12th Street and Madison Drive NW June 24 through 28 and July 1 through 5. Festival hours are 11 to 5:30, with evening concerts at 7 on June 25, 26, 27 and July 1, 2 and 3. Free admission. Incidentally, volunteers are needed: call 287-3467.

NATIONAL FOLK FESTIVAL--44th annual at Wolf Trap Farm Park July 16 through 18. More than a hundred performers celebrating traditional music and dance. Admission is $4, $2 for those 6 to 12, and there are evening concerts. Call 296-5422.

TAWES SUMMER SERIES--The University of Maryland takes care of summertime Tuesdays. Every week it features a different performer, artists from all walks of art: dancing, music, drama. It's already under way, with Leonard Rose on cello June 1. Tickets vary: call 454-2201.

OLD TOWN ALEXANDRIA FESTIVALS are Saturdays throughout the summer at Market Square on King Street. Usually 10 to 5 and featuring the arts and foods of different countries and cultures. Next one, the Mid-Eastern festival June 19. Free admission. Call 838-4343.

SUNDAYS AT FORT WARD--Civil War relived through battle reenactments, lectures, demonstrations and concerts. Free, Sundays 1 to 4 through August 8 at 4301 West Braddock Road. Call 838-4848

BALTIMORE'S SHOWCASE OF NATIONS--Weekly ethnic festivals featuring the food and music of various cultures at Center Plaza, Hopkins Plaza or Inner Harbor. Weekends noon to 8 starting with the June 5 and 6 Lithuanian Festival at Center Plaza. Free admission. 301/837-INFO.

GLEN ECHO PARK--Beginning June 13, The Glen Echo Dance Theater Company host its summer festival, the Chatauqua Season, inviting area dance companies to perform in the Spanish Ballroom Sundays 1 to 4. Admission, $2. Also on the weekends in the Ballroom there's dancing: Fridays it's square. Saturday it's folk and Sunday it's ballroom dancing. Admission is $3.

Summer Sunday afternoons there are free concerts in the bumper car pavilion, 1 to 5, with music from folk to jazz, while artists and craftsman show in the park noon to 6. The carousel is going (rides are 25 cents) and Part-Art, a free walk-in activity for kids, is in the Cuddle Up pavilion, where children are invited to mess around with paints and other crafts. All on MacArthur Boulevard, two miles inside the Beltway. Call 492-6282.

OPEN AIR MARKETS--Two of the largest: the D.C. Open-Air Farmers' market starting June 1 and continuing through December 23. Saturdays 9 to 3 and Tuesdays and Thursdays noon to dusk. Admission free at the RFK Stadium parking lot number six, at Benning Road and Oklahoma Avenue NE. (638-7300, ext. 202.)

And Baltimore's farmer's market is June 27 through December 19, at, appropriately, Market Place. Open Sundays 8 to sell-out. Two blocks from the Inner Harbor between Baltimore an Lombard streets.

BOOMERAND FESTIVAL--returns for the ninth year to the Polo Fields in West Potomac Park Saturday June 12. Free.