Rascally knaves and many another 16th-century aficionado will be role-playing at the 11th annual Maryland Renaissance Festival beginning Saturday on Crownsville Road five miles northwest of Annapolis. More than 200 groups will perform, including the Bedlum Mud Beggars, the Engelburt Hump Master of Turtles Racers, the New Riders of the Golden Age Jousters and thespians Shakespeare Skum. Take it all in, while you also take in the works of about a hundred craftsmakers, and foods such as steak on a stake, knaves sandwiches, mead and beer. Admission is $9.50 for adults, $8 for 12 to 15 and seniors, $3.50 for 5 to 11. It's open 10:30 to 6 Saturdays and Sundays (and Labor Day) until September 27. Take U.S. 50 to Rte. 450 east via Rte. 3 north. Exit on Crownsville Road and follow the signs to the festival site. 301/266-7304.

There's more in the way of time warps with the 3d U.S. Infantry (the Old Guard) and their colonial reenactment and parade, or what is known as a tattoo. It's 6 to 7 Saturday, featuring the Army Fife & Drum Corps and Commander-in-Chief's Guard. Afterward, you can take pictures of the boys in uniform. It's free at Fort Washington. Call 475-0856.


Some things never change and it's a good thing. Like these fairs:

The Montgomery County Fair kicks off Friday in Gaithersburg and along with the daily livestock shows, 4-H exhibits and arts and crafts sales, there'll be music by country group Highway 101 at 7 and 9 Friday and country vocalist Judy Rodman at 7 and 9 Saturday. Take Metro's Red Line to Shady Grove and Ride-On Buses No. 55 or 59 (60 cents) will take you to the fairgrounds, or park at Lakeforest Mall and take the shuttlebus (50 cents roundtrip). Friday and Saturday, admission and parking are free (and ride all the rides for $8); Sunday it's $1; Monday through Saturday it's $3, $2 for 12 to 15 and parking is $2. 926-3100.

The Howard County Fair winds up this weekend at 2210 Fairground Rd., off Rte. 144 in West Friendship, with concerts, parades and horse and livestock shows Friday and Saturday. 301/442-1022.

Join the urban-county cowpersons at the Arlington County Fair. Along with food and craft competitions, there'll be pony rides, a pet farm and a softball game pitting the Republicans against the Democrats at 4 Saturday. It's 10 to 9 Friday and Saturday, noon to 5 Sunday at the Thomas Jefferson Community Center, 3501 S. Second St. 558-2165.


Sitting on the dock of this side of the Bay is the fourth annual North Beach Bay Festival, noon to 6 Saturday with music, antique and craft displays and pet crab races. Speaking of seafood, there'll be crabcakes, steamed crabs and clams and lots more. Take Rte. 4 east from the Beltway 14 miles to Rte. 260 east, then nine miles to the Bay. Make a left on Rte. 261 (Chesapeake Avenue) a half-mile into town. Call 301/855-6681.


It takes a serious body to compete in the Crystal Light National Aerobic Championships, and Arlington's Ballston Common will be full of them 11 to 6 Saturday and Sunday. More than 150 athletes will sweat it out in three-minute routines set to music in individual, mixed couples and team competitions. It's free. Call 243-8088.


Christmas is 126 shopping days away, but that's not stopping WETA from celebrating now with their Christmas-in-August concert. In a program that guarantees to beat the heat, deejay Bill Cerri will present the Air Force Band at the Sylvan Theater at 8 Friday. It's free. WETA-FM (90.9) will broadcast the concert live. Call 998-2790 or 998-2800.


Go north for fun at Williamsport's C&O Canal Days this weekend. The festival offers an arts and crafts show at noon Friday, an ox roast ($3.50) followed by a street dance at 5 Friday, a 10K race at 8:30 Saturday (register at 7:30; $7), plus trolley and airboat rides, interpretive walks, bluegrass bands, an antique car show and a jousting tournament. Take I-270 to I-70 west to I-81 south to the Williamsport exit and follow the signs. Call 301/223-7711 on Friday or 301/223-6047 on Saturday.


There'll be 900 automobiles at the Laurel Lions annual auto show 8 to 5 Saturday, alongside food, entertainment and a flea market. $3. 490-7782 . . . more autos, of the antique kind, will be on display at the National Capital Trolley Museum on Bonifant Road (between Layhill and New Hampshire, north of Wheaton) 1:30 to 3:30 Sunday. 384-9797 . . . the Baltimore Summer Antiques Fair is noon to 9 Friday and Saturday and noon to 6 Sunday at the Convention Center, downtown at the Inner Harbor. $3.75 for all three days. 301/659-7000.


All of these hot flings are free, unless noted: the Dan Reynolds Jazz Ensemble plays at 7:30 Sunday at the Round House Theater, 12210 Bushey Dr., Silver Spring. 468-4172 . . . the jazz of Stanley Turrentine and the James Zimmerman Quartet fills Fort Dupont 8:30 Friday. 426-7723 . . . Rock 'n' roll with Flash Gordon and the Rockets at 7 Sunday at the Leesburg Courthouse. The evening includes '50s and '60s dance and costume contests. $2. 703/777-0574 . . . at Fort Hunt (on the GW Parkway south of Alexandria) the U.S. Navy Country Current Band performs at 7 Saturday. 285-2598 . . . A piano recital with Jin-Lan Ruey is at 8 Friday at Montgomery College's Rockville campus music recital hall, 51 Mannakee St. 279-5209 . . . the Montgomery County Barbershop Chorus harmonizes at the Friendship Heights Community Center (South Park and Friendship Boulevard in Chevy Chase) at 7 Sunday. 468-4232.