The five-year Bicentennial celebration winds up next weekend with a star-spangled hooraw at Yorktown, where His Brittanic Majesty's world turned upside down when Gen'l. Cornwallis surrendered to Gen'l. Washington on Oct. 19, 1781. Which means this weekend is the time to get away to the sleepy little (population 300) village where, as Nick Mathews says, "the victory and the freedom was won." Nick knows all about it. He and his wife, Mary, immigrants from Greece who met and married in New York, have long devoted their considerable energies to preserving the site and memory of the Battle of Yorktown. Among other things they donated the land for the Yorktown Victory Center and have for years paid the village street-light bill out of their own pockets. They also run the best restaurant between Washington and Norfolk, namely, "Nick's Famous Seafood Pavilion," which is reason enough for the two-hour trip. They're fierce patriots and fine folks, and a native-born visitor may come away from a conversation with Mr. and Mrs. Mathews uneasily trying to recall when was the last time he did something for his country. But, like we said, next weekend the place will be so packed as to be beyond approach. There aren't many streets in Yorktown, and the main one will be blocked off for the legions of dignitaries and sensation-seeking press people. There will hardly be a bathroom, much less a motel room, to be had that side of Richmond. So slip on down beforehand and see the Victory Center and roam the battlefield and reflect, if you like, upon how narrow-run a thing it was for the ragtag Continentals. The way to get there from Washington is to leave I-95 at the U.S. 17 South bypass below Fredericksburg. This takes you through some fine countryside and into Tappahannock, which Weekend sincerely and forever will regret having once slandered. It still is pretty tacky along the main drag, but turn left at the first traffic light and wander around the lovely town before returning to 17. If you want to get the full flavor of the place, seek help or advice from any citizen you encounter and said citizen will float you in kindness. Tappahannock also is notable for having the only gas station in Virginia where you can legally buy and drink a cold beer. To find it, ask the abovementioned citizen. To find Nick's Famous Seafood Pavilion, look to the left as you cross the York River Bridge. BICENTENNIAL THINGS IN ALEXANDRIA, H.M.S. Ariadne, a British anti- submarine and aircraft warship on its way to Yorktown, will be docked and open to the public at the Franklin Street pier on the Alexandria waterfront this Saturday, Sunday and Monday, 2 to 5. Also en route to Yorktown: re-created French and American troops march across the Woodrow Wilson Bridge October 15 at noon. The 4,000 participants then parade through Old Town at 1:30 and stage a mock battle at 3:45. (Call 838-4994.) IN YORKTOWN, those troops arrive October 16 to reenact the beginning of the encampment. Demonstrations of cannon firings and 18th-century military camp life are at Colonial National Historical Park October 16-19. Historical background is provided through multimedia displays, now through December, at the Victory Center. Daily 9 to 5; admission $2, $1 for senior citizens and 75 cents for kids. IN CHESTERTOWN, Maryland, Washington College begins a year-long bicentennial celebration this Saturday. Maryland Governor Harry Hughes begins it at 10 with a procession leading to a flag- raising ceremony from the Hynson-Ringgold House. Art exhibits and athletic activities and the first in a series of lecture Behind the Sculpture in Lafayette Square." Saturday 2 to 5 at St. John's Episcopal Church, 16th and H streets NW.