Today and tomorrow the Music Division of the Library of Congress is going to observe Founder's Day, as it does every year. The founder in this case is Elizabeth Sprague Coolidge, who was born in Chicago on Oct. 30, 1864.

When she was 49, Mrs. Coolidge motored, as they called it those more leisurely days, from one of her summer music festivals in Pittsfield, Mass, to southern Virginia and then back to Washington. " There, " she later wrote, " we were invited to take luncheon at the ' Round Table, ' a gathering of the Chiefs of Division in the Library of Congress, president over by its wonderful librarian, Dr. Herbert Putnam. I happened to be seated next to a certain Dr. Moore, who, while chatting about my musical affairs, rather pointedly asked me if I might not consider giving some such music to the Library of Congress. "

Out of that " chatting " came the Coolidge Auditorium of the Library of Congress, the Coolidge Foundation in the Library and a history of commissioning new music that has no parellel in history. No wonder the Library marks Founder's Day each year with a special concert.

There is a fine twist to this year's program: In it, Paul Callaway will conduct a chamber orchestra and chorus, with soloists, in music written at the request of Mrs. Coolidge or by those in charge of the foundation she established.

Ildebrando Pizzett's Epithalamium, with which the program opens, dates from 1939. It is for solo voices, small chorus and orchestra. Ottorino Respighi's 1927 Trittico Botticelliano, which comes next, is for chamber orchestra, each of its three sections inspired by a famous Botticelli painting.

Ned Rorem's " Letters From Paris, " a recent work, calls for the same forces as the Pizzetti. It sets texts from letters by Janet Flanner that appeared in The New Yorker magazine over a period of 20 years, carrying the signature "Genet. "

There will be those in the audience at the Library on Sunday afternoon and Monday evening who remember Mrs. Coolidge, from the last years of her life - she died on Nov. 4, 1953, at the age of 89 - when she was a regular visitor to the concerts she established.

This Elizabeth Penn Sprague Coolidge was a formidable character. According to her niece, Lucy Sprague Mitchell, Elizabeth stood 5 feet, 11 inches, and close-friends described her figure, at middle age, as " slightly Amazonian. "

When it came to musical accomplishment, Mrs. Coolidge was decidedly on the Amazonian side. Alfredo Casella, one of the leading composers of 20th-century Italy, called her " One of the noblest figures in the history of musical patronage. " But after citing her gifts and accomplishments, he noted, " Life with Mrs. Coolidge was not easy. Her guests were supported to hold themselves at her disposal all day long and even late at night if she so desired. She was a woman of really phenomenal physical constitution; although she herself was never tired, her guests were often completely exhausted. "

The exhaustion occured during the festivals of new music Mrs. Coolidge began to present in European capitals soon after inaugurating them at her festival site on South Mountain near Pittsfield, Mass. From her first foreign festival in Rome in 1923, Mrs. Coolidge led her colleagues - for she often played piano in various chamber works - in London, Paris, Vienna, Berlin, Brussels, Amsterdam. Frankfurt, Prague, Budapest, Moscow, Venice and Naples.

My own first reason for vast gratitude to Elizabeth Sprague Coolidge came in 1835 at the University of Chicago when she sponsored the Pro Arte Quartet in visits to a number of American university campuses, where they played the complete cycle of Beethoven quartets. This was the first of numerous times I enjoyed these quarters under various Coolidge auspices.

Even before coming to Washington, I heard music from the Coolidge Auditorium at the Library of Congress when the festivals began to be broadcast. It was a sign of this remarkable woman's progressive outlook that she urged the broadcasts at a time when as she said, " some of the artists and even Dr. Engel himself (Lebman Engel, then chief of the Music Division) were not enthusiastic. Some conservatives feared this radio competition with personal appearances; but I believe that it ultimately increased their audiences. " Solid foresight!

Imagination and a gift for looking into the future had often marked the somewhat autocratic, talented Elizabeth. She was playing in piano recitals by the age of 12, gave her own first solo concert at 18, and played with the Thedore Thomas Orchestra, the forerunner of the Chicago Symphony, at the age of 29, seven months before her only child, Albert Sprague Coolidge, was born.

As for taste, sensitivity and awareness of new voices in music, her list of commissions is not only proof, it reads like a musical hall of fame of this century. It includes, to name only some of the most famous items:

Bartok, Quartet No. 5, Copland, Applachian Spring, Piano Quartet; Hanson, Quarter; Hindernith, Herodiade; Loeffler, Canticle of the Sun; Milhaud, Jeux de Printempts; Piston, Partita; Ravel, Chansoms Madecasses; Schoenberg, Quartets 3 and 4; William Schuman, Night Journey; Stravinsky, Apollon Musagetes (now called Apollo) - and dozens more.

Aaron Copland has given a vivid description of just how his most famous composition came into being: "Appalachian Spring owes its origin to the fact that in 1942 the well-known music patron, Mrs. Elizabeth Sprague Coolidge, was taken to a performance of Martha Graham's Dance Company. Stimulated by what she saw, Mrs. Coolidge, with typical generosity, decided to commission three composers of Miss Graham's choice to compose three ballets for her. Martha chose Hindemith, Milhaud and myself."

It is happy decision on the part of Donald Leavitt, the new chief of the Library's Music Division, to present three Coolidge works on this year's Founder's Day concerts. It is a custom that could be followed for many years without coming to the end of a superb list of works deserving to be heard again and agian. Unfortunately, it is no longer possible to hear some of those works as they were first heard, since some of them, like the Piston Partita, the Loeffler "Canticle of the Sun," and compositions by Marcel Grandjany, among others, call for pipe organ. When Mrs. Coolidge planned the auditorium that carries her name, she included an organ designed by Ernest Skinner. Some years ago, however, that instrument was removed from the library and sold to Holy Trinity Church, Georgetown, where it is at the moment in the process of being revoiced and put into first-class shape as a part of the larger instrument in the church.

It would be possible today to place a new organ of the finest contemporary design in the Coolidge Auditorium. Such an addition, so much in keeping with her original plans, would be an invaluable way of restoring to the auditorium an element which she thought vital to its ultimate aim, which she put this way:

"I have wished to make possible, through the Library of Congress, the compositon and performance of music of ways which might otherwise be considered too unique or too expensive to be ordinarily undertaken. Not this alone, of course, nor with a view to extravagance for its own sake; but as an occasional possibility of giving precedence to considerations of quality over those of quantity; to artistic rather than to economic values; and to opportunity rather than to expediency."

To hear again much of the music she caused to be written in the surroundings she provided for it is the finest continuing tribute that can be paid to the memory of Elizabeth Sprague Coolidge.