A SINGULAR NEW honor is coming Aaron Copland's way tomorrow at 8 p.m. On the front lawn of the U.S. Capitol, with the blessing and under the sponsorship of the Congress of the United States and the Secretary of the Interior, Copland will conduct the first in a new series of concerts by the National Symphony Orchestra.

Tomorrow is Memorial Day, and Copland, whose life has been filled with all of the highest honors that can come to an American musician, seems a singularly appropriate choice to inaugurate this new venture in the musical life of this city and country. There will be four of these concerts this summer, three on the three big holidays, that mark our holidays months: Memorial Day, the Fourth of July and Labor Day. Copland will be followed, on Independence Day, by the National Symphony's music director, Mstislav Rostropovich, and on Labor Day by Erich Leinsdorf. The fourth concert, in mid-August, is scheduled to be conducted by Arthur Fiedler, who has said that unless his health makes it impossible - and he has been astonishing his doctors and friends at regular intervals lately - he will be here with bells on.

Copland, the man born in Brooklyn 80 years ago next Nov. 14, has become a symbol, as the author of not only much of the finest music ever written by any American, but also some of our most characteristically American music. It is easy to say that American music is music written by an American. But there are composers, good ones, in this country, whose music does not carry that unmistakable flavor, texture, rhythm, whatever it may be, that says, "This music could not have been written by anyone but an American," in the way that some music is instantly recognizable as German, French, Spanish, or English.

But Copland, out of his Brooklyn roots, and studies in New York City and in Paris, grew up to create a series of works in which the very way the notes are put together and follow one another proclaims them as the music of a native of these United States.

The program Copland has planned for tomorrow night is framed with two of those works: the "Fanfare for the Common Man," which will open the program - and somehow these holidays seem especially the times when the common man is the central figure - and the "Lincoln Portrait," with which the evening will come to a close. This composition, which has become one of the Copland's best known and most frequently performed, calls for a narrator. Tomorrow night, when it is time for the "Lincoln Portrait," Copland will turn over the baton to guest conductor Gerhardt Zimmerman of the St. Louis Symphoney, in order to take on the duties, and pleasures, of the narrator himself.

In between these two particularly symbolic works, Copland will conduct his own Clarinet Concerto, written for Benny Goodman. His soloist will be one of the National Symphony's superstar first-chair players, Loren Kitt. From the non-Copland repertoire, there will be Samuel Barbar's brilliant overture for the "School for Scandal," which we may hope will not suggest anything untoward to any listener by being heard in that unusual setting.

And there will be Gershwin, to match the festive mood of the day, the sounds of "An American in Paris," certainly one of the greatest holiday pieces to come of this country. For an example of perfect matching of music to moment, Copland will also conduct "Decoration Day" by Charles Ives, a work which Copland has called "unquestionably among the finest works ever created by an American artist." His description is no more than the work deserves and we are just plain lucky that Ives did choose to memorialize in music of extraordinary power four of our great national holidays: "Decoration Day," which, for any of you who may wonder at the label, is what Memorial Day used to be called in the years following its institution as a time for visiting the graves of fallen soldiers and decorating them with fresh flowers and flags; "Fourth of July," which we must hope Rostropovich will conduct in five weeks; "Thanksgiving Day" and "Washington's Birthday." In these four remarkable works, Ives evokes memories of his own childhood in the lovely small town of Danbury, Conn., nestled among some of New England's loveliest hills.

Picnic baskets, blankets, and families, along with singles and couples should start filling up the great green spaces west of the Capitol late tomorrow afternoon. The orchestra will be amplified, playing under a tented roof, sending its music out to the thousands who will cover the grass Mall. What a glorious spot for the young man from Brooklyn! CAPTION: Picture, Copland: Capitol conductor.