SOME OF the newest Christmas music in town turns out to be some of the oldest each year, when the Folger Consort presents its annual holiday concert. Most Christmas programs are predictable (and widely loved for their predictability), but not this group's.

Its repertoire covers an enormous time- span -- five centuries from the 12th to the 17th -- when Christmas, still undiscovered as a stimulus to commerce, was celebrated intensively by artists and particularly by musicians. Little masterpieces of unfamiliar Christmas music abound in this period, and each year the Consort unveils an array of them in sparkling performances.

For this year's Christmas concerts, which begin Saturday afternoon and continue through a week from Sunday, the Folger Consort will be joined by a noted English vocal group, the Tallis Scholars, in a program that includes such early-music favorites as "Greensleeves," "Riu, riu, chiu," "Joseph Lieber" and "Es ist ein Ros entsprungen" as well as less familiar but exquisite music by such composers as Heinrich Isaac and Tomas Luis de Victoria.

Live cassette recordings of the Consort's '83 and '84 Christmas programs are available at the Folger Library, in person or by mail order. Both concerts present vintage performances of well-selected programs.

THE FOLGER CONSORT -- "Nowell Sing We" (1983 Christmas program) and "Sing Heigh, Ho! Unto the Green Holly!" (1984 program) in audio cassette limited editions; appearing at the Folger Shakespeare Library, Saturday at 3 and 8:15, Sunday at 3 and 7, Monday and December 20 at 8, December 21 at 3 and 8:15, December 22 at 3.