It's that time again:

D.C.'s megafest, Adams Morgan Day, is noon to 8 Sunday along 18th Street between Florida Avenue and Columbia Road. There'll be two big 6 p.m. concerts: fusion jazz from Pieces of a Dream at the 18th and Columbia stage, and go-go from Experience Unlimited at 18th and Florida. All day, more than 500 entertainers perform on five stages and 250 vendors sell anything you can imagine or eat. Don't even think about parking; take Metro's Red Line to Dupont Circle, walk north on Connecticut, bear right on Columbia and follow the music. 745-0179.

Everybody knows Maryland is for crabs, but it's especially true at the Maryland Seafood Festival noon to 8:30 Friday, Saturday and Sunday at Sandy Point State Park near Annapolis. Besides the crabs, there'll be oysters, shrimp and fish and big-band music along with a treasure hunt for kids. Admission's $4 for adults, free for kids under 14 with an adult; thereafter, it's pay-as-you-eat. Call 301/268-7682.

The 10th annual Takoma Park Folk Festival harvests fruits of the local and international artistic community 11 to 6 Sunday at Takoma Park Intermediate School, 7611 Piney Branch Rd. In all, 25 groups of musicians, dancers, storytellers and actors will perform, including Central America's Izalco, singer-songwriters Cathy Fink and Marcy Marxer and the Takoma Repertory Theater. Hummus sandwiches, hot dogs, Vietnamese dishes and more will be for sale. Take Metro's Red Line to Takoma and hop a free Ride-On shuttle. 270-5367.


The second annual Black Family Reunion Celebration sets up on the Mall (between 10th and 14th) with 13 pavilions of educational family activities -- exhibits, puppet shows, storytelling -- from 9:30 to dark Saturday and Sunday; speakers include Jesse Jackson, actress Esther Rolle and Betty Shabazz, widow of Malcolm X. Free concerts follow: at 6 Saturday, it's singer Melba Moore, Force M.D.'s, and singer Beau Williams, then at 6 Sunday it's the Staple Singers and a dozen other local gospel choirs. The celebration's sponsored by the National Council of Negro Women Inc. 684-5733.


One of Maryland's oldest fairs, the Prince George's County Fair, is this weekend at the Equestrian Center in Upper Marlboro. Friday highlights include concerts by the Hubcaps at 8 and 10 with a laser light show in between. Saturday at 1 it's the Atlantic Coast Arm Wrestling Championships. Sunday at 1, the Seldom Scene gives a bluegrass concert. Some mighty big porkers will be hamming it up in daily pig races. It's $4, $3 for seniors, $2 for kids 7 to 12. 952-0270.


The Hagerstown Air Show and Maryland State Balloon Championship swoop down to Washington County Regional Airport (off I-81) 1 to 4 Saturday and Sunday. Higher-ups include wingwalkers, the 350-mph Bud Light Micro Jet, the Redhawks precision flying team and pilot Bob Hoover, who does his stunt flying in big corporate jets. It's $6, free for 11 and under. Besides the airshows, 30-odd balloons jockey toward the "hare-balloon" in precision contests at 7 a.m. Saturday and 7 a.m. and 5 p.m. Sunday that are free to watch. 301/739-1993.


More than a dozen bands converge on Blob's Park (take Rte. 175 east of the Baltimore/Washington Parkway) in Jessup from 11 to 11 Saturday for the Potomac River Jazz Club's 17th annual dixieland and old-time jazz picnic. While you listen to the Buck Creek Jazz Band, Federal Jazz Commission and Imperial Palms Orchestra (among many others), you can peruse a flea market and munch on bratwurst and burgers for sale. Your $10 at the gate (under 16 free with adult) includes soda and beer. 698-PRJC.


It's no run-of-the-mill weekend at Colvin Run Mill's "Autumn Traditions" celebration. From 11 to 5 Saturday and Sunday there'll be milling demonstrations and crafters making everything from baskets to furniture to cider. Both days the Fairfax Symphony Bluegrass Ensemble and folk duo Side By Side perform. Sunday at 4, there's folk music by Legacy. It's $4, $2 for kids 3 to 15; five miles west of Tysons Corner on Route 7 (off Beltway Exit 10B). 941-5000.


The dog days make way for a felinely fall at the National Capital Cat Show. From 10 to 4 Saturday and Sunday, more than 500 cats show off at the Thomas Jefferson Community Center (3501 S. Second St.) in Arlington. There'll be exhibitions, competition and purebred kittens and purr-oducts for sale. Admission is $4, $2 for under 12. 256-6014.


The National Kidney Foundation's Maryland State Chili Cookoff begins at 11 Saturday at Summit Hall Farm Park on Rte. 355 in Gaithersburg. $2. 337-6600 . . . Pianist Arthur Greene, winner of the 1986 William Kapell Competition, performs at 4 Sunday at the Lyceum, 201 S. Washington St., Alexandria. Free. 960-0616 . . . the last of Baltimore's summer "Showcase of Nations" festivals is the Irish Festival, noon to 10 Saturday and Sunday at Festival Hall, Sharpe and Pratt streets. 301/837-INFO.


The National Archives begins an 87-hour vigil at 6 Sunday to celebrate the 200th anniversary of the signing of the Constitution, with patriotic and historical films, music and dramatic performances. Call 523-3000 for details.

Madison Day festivities fill the Octagon from 6 to 9 Saturday with events that President and Mrs. Madison were accustomed to, including the colonial children's play "Sunrise in Philadelphia," a costumed march from the White House to the Octagon by the Fort McHenry Guard and a candlelight illumination ceremony. It's $5 for adults, $3 for kids under 16 at 1799 New York Ave. NW. Call 638-3105.