The festival season is upon us! Take your pick and take the family:

If it's water you want, the Alexandria Red Cross Waterfront Festival is what you need. To scrape the surface of the six-block event: Sailing ships will be coming into the city marina throughout the three-day fest, a remote-control model sailboat regatta is 10 to 1 Saturday and the Best Friend of Charleston, an 1840 replica passenger train, will be hauling folks for free from noon to 4 Saturday. Plenty of concerts, including the Carolina Beach Music Festival at noon Sunday, will keep it all in tune, and a fireworks display will close things brightly 9:30 Friday and Saturday. Free shuttle buses operate Saturday and Sunday from Metro's King Street station to Fairfax and Pendleton streets. The festival is 6 to 10 Friday, 10 to 10 Saturday and 11 to 6 Sunday; $2 per day, kids under 12 free. 838-5005.

Potomac Riverfest '87 splashes on this weekend at Anacostia Park, between the 11th Street and Pennsylvania Avenue bridges, with boat rides, ethnic foods, clowns, arts and crafts demonstrations and five performance stages, including the music and dance of "District Cultures," 14 groups doing everything from doo wop and motown to African and Middle Eastern dance. Admission is free. Call 745-3737.

'Tis the lark? 'Tis the Chesapeake SummerArts program of St. Mary's College with the opening of the Maryland Shakespeare Festival. It starts at 8 Friday with "The Comedy of Errors" and the drama continues every Friday, Saturday and Sunday through August 9. ("The Rainmaker" runs July 3 to July 19 and "The Beggar's Opera" July 24 to August 9.) Tickets are $7 to $9; follow the signs from Rte. 5 south to the courtyard of Farthing's Ordinary (that's an inn) in St. Mary's City. Come early Saturday and you can watch the annual Lord Baltimore Jousting Tournament, at 1:30 adjacent to the Visitor's Center. Call 301/862-0243. A MARCH IN TIME Put one foot in front of the other and soon you'll be raising money for a good cause. That's the plan the Metropolitan Chapter of the Arthritis Foundation has with their 10K walk-a-thon Saturday. Registration is 10 to 11:30 at the Washington Sheraton Hotel and the walk begins at noon. The 6.2-miler winds through Rock Creek Park and back to the hotel, which is near Metro's Woodley Park/Zoo station on the Red Line. It's $10 to walk, or get a sponsor sheet by calling 276-7555. CRUISE OVER Need a boat? Or do you just like to dream? Either way, the Baltimore In-Water Boat Show is 10 to 7 Friday and Saturday and 10 to 6 Sunday at the Baltimore Marina Center, 2701 Boston St., with hundreds of new and used boats in the water and on land. It's $5, $2 for 12 and under. Call 301/342-3003. HI-HO SYLVESTER! Yo! Sylvester Stallone is expected to suit up for a match from 4 to 6:30 Sunday at the Potomac Polo Club, River and Hughes roads in Poolesville, 15 miles west of Beltway Exit 39. It's $5, kids under 12 are free; this benefits the National Center for Therapeutic Riding. Call 881-5040. EASTERN NOTES We speak many different languages, except when it comes to jazz. The Soviet Union's leading traditional jazz group, the eight-member Leningrad Dixieland Jazz Band, stops in town on its first tour of the hemisphere at the Baird Auditorium of the Smithsonian's National Museum of Natural History. Tickets for the Smithsonian Resident Associate concert at 7:30 Sunday are $12. Call 357-3030. FREEBIES What are summer weekends without free concerts? While you're thinking, here are a couple to consider:

Concerts on the Canal brings music to Georgetown 1:30 to 4 every other Sunday beginning this weekend at the Foundry Mall between 30th and Thomas Jefferson NW. The program commences with song and dance by the Irish Breakdown and the Lisa Rich Quartet. Call 862-1300.

Young stars of tomorrow shine at the Library of Congress Summer Chamber Festival at 8 Friday in a program that includes works by Mozart, Schumann and George Rochberg; the festival continues same time, same place June 16, 19 and 23. It's at the Coolidge Auditorium of the Jefferson Building, First and Independence SE, one block north of Metro's Capitol South station. Call 287-5502. OLD MOVES See how far we've come and gone at the following events:

Remember the little train that could? The Chesapeake and Allegheny Steam Preservation Society does and this weekend they'll show how it was done -- with working scale models. You'll also be able to climb aboard the cars, which are about two-by-ten feet, and catch a free ride. It's noon to 4 this Sunday and the second Sunday of every month through November, at Leakin Park at Windsor Mill Road and Forest Park Drive in Baltimore. Call 483-2985.

Sully's 14th annual Antique Car Show boasts 350 antique and classic cars, an old fashions show, the Fairfax City Dixieland Band and a flea market with more than 100 vendors. It's 10:30 to 3:30 Sunday, $4 ($2 for kids 3 to 15) at Sully Historic Site on Rte. 28 in Chantilly, four miles south of the Dulles Toll Road. 437-1794. REST OF THE BEST Flag Day celebra- tions at Fort McHenry include a parade of flags and fireworks by the same Grucci family that wowed Washington last weekend. It's 5 to 10 Sunday. Free. 301/563-FLAG . . . the 16th annual Mountain Heritage Arts & Crafts Festival between Harpers Ferry and Charles Town, West Virginia is 10 to 6 Friday, Saturday and Sunday. $4, $2 for kids 6 to 15. 800/624-0577 . . . a free program of Hawaiian music, hula dancing and a lei-draping ceremony is 1 to 2:30 Sunday at the Statuary Hall of the U.S. Capitol. 839-5543 . . . The Japan-America Society's 30th anniversary bazaar is 12 to 5 Sunday at the Georgetown Visitation School, 1500 35th St. NW, with food, crafts and performance. $1. 289-8290.