On the Feast of Fools, shortly before Christmas, people used noisemakers to frighten away demons in the busy streets of 12th-century Paris. "Whoever is sad will be made happy by these rites," rowdy students sang in a Latin jingle. The song is used by the Folger Consort to open this year's Christmas concert, which explores many curious byways of music and folklore.

The program visits medieval England, where the boar's head -- essentially, like holly and mistletoe, a pagan symbol -- became a Christmas motif. The performers spend some time in 17th-century Mexico, where Indian composers produced Christmas songs purely European in style and more elaborate than those being composed in Spain. They sample the smooth, deceptive simplicity of Renaissance Parisian music, the sweet complexities of Heinrich Schutz, the elegant cadences of Sweelinck's organ music, working out variations on a simple carol.

Aided by a host of guest artists, the Folger Consort has produced a fine sequence of works that will be unfamiliar except to specialists, but immediately and spontaneously enjoyable. The music breathes the joy and freshness of a world where Christmas was still a mystery undiscovered by market analysts. It will be repeated daily through Sunday -- and for those who want to take home a sample, the Consort has tape cassettes of last year's superb concert on sale in the lobby.