SIX WORLD premieres and a dozen and a half Washington firsts will help to make the 38th annual American Music Festival opening this evening at the National Gallery of Art a major event to be enjoyed over the next nine Sundays.

Richard Bales has brought together a list of major events that seem likely to make this festival one of the most important in his entire career as music director of the National Gallery.

John LaMontaine, whose piano concerto won him the Pulitzer Prize in 1958, will be heard in a new flute concerto on April 12. Another major event will be the first hearing of a large symphony by John Powell (1882-1963), one of the signally important figures in the history of American composers. Several years ago Roy Hamlin Johnson of the piano faculty of Maryland University edited and published a critical edition of Powell's tremendous Piano Sonata Teutonica. Johnson's work filled a significant gap in our knowledge of major American composition.

For the past several years, Johnson has been at work on a definitive performing edition of Powell's Virginian Symphony, a large work that has never been performed. Bales will give the symphony its premiere on the final program of the festival on May 24.

Three Washington composers will enjoy first performances during the festival: Genevieve Fritter's Poem for flute and orchestra will be heard on May 24; Thomas Beveridge's Dialogues on April 12; and Russell Woollen's setting of Psalm 148 on the May 10 program of the Paul Hill Singers.

One of the extraordinary novelties in the festival is the presence of the piano concerto by William Schuman. It is strange that so major a work by another of this country's Pulitzer Prize-winning composers has had to wait this long to be heard in this city.

The April 19 concert in honor of Aaron Copland's 80th birthday will present pianist Claudia Stevens in a program that includes the Washington premieres of six shorter pieces written to honor Copland. Ranking with Copland and Schuman is the late Roy Harris, whose String Quartet No. 1 will have its first local hearing.

Bales has consistently arranged the American festivals to ensure that both new and established American music is heard in a variety of idioms. This year's nine concerts include four by the National Gallery Orchestra, two piano recitals -- by Claudia Stevens and Sylvia Glickman -- chamber music by the Blair String Quartet from Nashville and the Huntingdon Trio, and a program by the Paul Hill Singers.

All of the festival concerts, like all the National Gallery concerts, are open to the public without admission charge, and are broadcast over Station WGMS each Sunday evening at 7.