This Monday, May 26, Americans will carry on a tradition that is more than 100 years old when they gather at Memorial Day services throughout the country.

Although Memorial Day officially is May 30, Congress approved legislation several years ago that brought Memorial Day under the Monday Holiday Day bill.

Memorial Day, still known in many areas as Decoration Day, Was first celebrated shortly after the Civil War, when a group of Union veterans urged that a special day be set aside to honor their slain comrades. One of the first celebrations was at Arlington Cemetery in 1868, on the grounds of Gen. Robert E. Lee's home, which had been confiscated by Union forces at the start of the Civil War.

For many years, the celebration was taken as an opportunity to herald the Union victory, but gradually, a call for national unity overcame the often bitter rememberances of the Civil War.

The holiday has had special significance for the military. In honor of the day, the Army ordered military posts to fly flags at half-mast until noon. The Navy followed a tradition of launching small, flower-laden boats in honor of men lost at sea.

Although traditionally set aside as a day to honor soldiers who have fought in the five major conflicts since the Civil War, Americans also have come to honor other loved ones.

In Northern Virginia, Memorial Day will be marked by military services, community festivals and parades. Among the activities scheduled Monday: ARLINGTON CEMETERY

At 11 a.m., ceremonies will be held at the Tomb of the Unknown soldier. Miss America will sing the national anthem just before the Armed Forces Full Honor Wreath Ceremony is held at the tomb. The ceremonies are free and open to the public. FORT BELVOIR

At 9 a.m., the 50 state flags will be raised, with the national flag raised to half-staff.

At noon, the traditional 21-gun salute will be fired, with the national flag hoisted to the top of the staff. Guests will then observe a minute of silence. The state flags will be lowered at 3 p.m.

The ceremonies are free and open to the public. Visitors should use the main gate (Pence Gate), off Rte. 1. ALEXANDRIA

A jazz festival in Market Square, 300 King St., in Old Town will be held from 11 a.m. to 5 p.m. The festival is free and open to the public. VIENNA

At 10:30 a.m., Vienna will hold special ceremonies at the Flint Hill Cemetery off Rte. 123 in Oakton. The ceremonies include a concert by the Vienna Band, the presentation of colors by the American Legion Post 180 drill team and an address by Vienna Mayor Charles Robinson. The ceremonies are free and open to the public. FALLS CHURCH

Falls Church will hold its 29th annual Memorial Day parade beginning at 3 p.m. at Southwest and Ellison streets. The parade ends at Cherry Hill Farm, 312 Park Ave. (adjacent to the Falls Church Municipal Building). The grand marshal of the parade is Miss Canada. In addition, there will be a festival from 9 a.m. to 5 p.m. on the grounds of Cherry Hill Farm, with handicrafts, food and beverages available. The parade and festival are free and open to the public. QUANTICO-DUMFRIES

At noon, ceremonies will be held at the Statue of Iwo Jima, in the triangle at the entrance to Quantico, Rte. 1 and Joplin Road.

Beginning at 2 p.m., the Buck Keys Post No. 28 of the American Legion will sponsor ceremonies at 110 Stage Coach Rd., Dumfries. Retired Lt. Gen. F. L. Wieseman will speak. Offices closed

County government offices, schools and courts in Arlington, Alexandria and Fairfax County will be closed Monday for Memorial Day.

Regular trash collection schedules will remain in effect.