Benjamin Britten festival will be presented in the Unitarian Church of Washington on three successive weekends this month. On Friday and Saturday at [WORD ILLEGIBLE] p.m., there will be performances of "Let's Make an Opera." This is the only one of the three offerings for such an admission will be charged. On Dec. 18 at 8 p.m. four Britten choral thanks will be given, and on Christmas Eve at 11:30 p.m. his cantata, "St. Nicolas" will be sung. Vera Tilson, music director of the church, is in charge of the three programs.

Jacqueline Fontyn, Belgin composer and professor of composition at the Royal Conservatory in Brussels, will be in Washington for a week beginning Wednesday. On Dec. 18 her Nonet for woodwinds and strings will be performing in New York City in the Carnegie Recital Hall.

The National Symphony's two annual competitions for young artists will be held in January. Deadline for entries for high school students is Jan. 2, with Jan. 27 the final date for college students. Complete details on repertoire, which is precisely the same for both age groups, are available for Mrs. Donald May Jr., chairman, 5931 Oakland Rd., McLean, Va., 22101.

Robert Bialek, president of Discount Records and Books, has commissioned a work for flute and orchestra from Alan Hovhaness. Its world premier will be given by Jean-Pierre Rampal and the National Symphony under Rostropovich in the spring of 1979.

The Hovhaness commission is one of a series Bialek has awarded in recent years. Others have gone to Alberto Ginastera for a guitar serenade, to David Amram and Robert Evett for bassoon concertos, and to Antal Dorati for an oboe concerto.

From Wolf Trap: The annual Carol Singing will begin at 4:30 p.m. Sunday in the Filene Center. Choirs, soloists and the Marine Band will be on hand for the festivities, which are free and may be cold.

The Shir Chadash Choir, formed recently by Herman Berlinski, will make its first Kennedy Center appearance Saturday at 5:15 p.m. in the Concert Hall. Berlinski will play organ solos and conduct the choir and audience in traditional Hanukah songs. There also will be exerpts from Handel's "Judas Maccabeus." Free tickets will be distributed at the Park Service desk in the Center.

At Howard University's annual Christmas choral program Saturday and Sunday, to be sung at 6 p.m. in Rankin Chapel, the soloist will be soprano Mattiwilda Dobbs, internationally-known opera singer, who is now teaching at Howard.