To observe the 150th birthday of Johannes Brahms, which comes this Saturday, the Library of Congress is staging an ambitious six-day Brahms celebration, beginning today.

There will be five nightly concerts, ending on Saturday, in the Coolidge Auditorium, with such famed performers as soprano Elly Ameling, violinist Gidon Kremer, pianist Charles Rosen and clarinetist Richard Stoltzman. The concerts are sold out, but the Library is putting together a waiting list in case there are cancellations. To get on the list, call 287-5502.

An International Brahms Conference for scholars will open at 8:45 a.m. Thursday with a keynote address by the eminent musicologist Karl Geiringer. All events are open to the public.

The Library also will display a selection from its collection of Brahms manuscripts and letters, which is believed to be the world's largest and finest. Among the manuscripts in the collection: the Violin Concerto, the Third Symphony, the piano quintet, the horn trio, the Handel Variations and many of the late piano pieces.

At tonight's concert, which starts at 8, pianist Detlef Kraus will play two recently discovered gavottes that Brahms wrote when he was 21. Kraus will also perform "Variations on a Theme" by Handel. Violinist Robert Gerle and pianist Marilyn Neely will play the G major Sonata, Op. 78, and the C minor Sonatensatz.

The remaining concerts will begin at 8:15, and be preceded each evening by scholarly lectures at 7.

On Wednesday, Neely plays the F-sharp minor Sonata, Ameling sings Brahms songs and Kraus plays the Piano Quintet with the Concord String Quartet.

Thursday duo-pianists Eden and Tamir play the Schumann Variations and the two-piano version of the Third Symphony, and Kremer performs the D-minor Sonata, Op. 108.

Friday Stoltzman does the F minor Clarinet Sonata, Kraus plays some short pieces, tenor Paul Sperry sings some songs and the Cantilena Chamber Players do the B flat major Sextet.

At the final concert, Rosen plays the Op. 119 piano pieces, joins Gerle and horn player Richard Todd in the Horn Trio and the Concerto Soloists of Philadelphia perform the Second Serenade.