On Sunday afternoon, July 29, they're going to re-fight the first Battle of Manassas (as the Confederates called it) or the first Battle of Bull Run (as the Northern side called it) that occurred with a tragic shedding of much blood back in 1861.

The reenacted battle, with 5,000 make-believe troops in replica uniforms of blue and gray, will be witnessed 125 years after the real encounter by as many as 50,000 spectators on a field north of Centreville, about five miles north of the actual Manassas Battlefield Park.

In a sense, the reenactment will be an enhanced replay of the Civil War centennial restaging of First Manassas on the actual battlefield in 1961. My family went, and what we saw was unforgettably dramatic.

Plans for the 1986 restaging -- touted as the largest battle reenactment ever held in this country -- were described at a meeting of the national capital unit of the Council on America's Military Past by Jack Thompson of Fairfax. Thompson is a director of the sponsoring nonprofit organization, the American Civil War Commemorative Committee, based in Culpeper, Va.

Worldwide interest in the Civil War was sparked by centennial activities between 1961 and 1965, Thompson said, and numerous reconstituted, if sham, Civil War military units have been formed and manned by hobbyists who sport uniforms, equipment and weapons made to the precise specifications of the originals. Those, he said, weren't widely available 25 years ago.

The new battle site is a 500-acre parcel off Virginia Rte. 28 between I-66 and U.S. Rte. 50 that is destined to become the Westfields corporate center. Thompson said its rough terrain resembles the original battlefield more than does the well-manicured battlefield park.

The number of artillery pieces on the field will equal the number on the original battlefield, Thompson said, and 1,500 artillery rounds and nearly 500,000 small-arms rounds (all blanks) will be expended. Participants will come from as far away as Australia and Germany.

Encampments, open to visitors, will begin on Thursday, July 17.

Tickets at a basic adult price of $3 will go on sale May 1. For information in the metropolitan area call 276-1861. Ultimate Garden Tour

On a more peaceful note, spring garden tours of the White House grounds will take place from 2 to 5 p.m. this Saturday and Sunday. Any visitor in line by the closing hour will be admitted.

The tours, hosted by White House guides and National Park Service rangers, include the Jacqueline Kennedy Garden, the ceremonial Rose Garden and the presidential commemorative trees. Some public rooms of the White House will be open. Outdoor, but not indoor, photos will be permitted.

The visitor entrance is on East Executive Avenue, opposite the Treasury, and can be reached only from E Street. More Terrific Tulips

The other day, Metro Scene mentioned some of the spectacular tulip gardens in the city. Since then, I've seen one other that's especially appropriate as well as beautiful: the beds at the base of the Netherlands Carillon, next door to the Iwo Jima Memorial in Arlington.