It may be the season to be jolly, but the musical mood was closer to Lent at the National Cathedral Saturday afternoon. In a sold-out concert billed as "The Joy of Christmas," the joint choral forces of the Cathedral offered somberness for the holidays.

Only the processional exploited the expansive acoustics, as vocal textures varied marvelously with the peripatetic choir. During the rest of the concert, the chorus seemed tired and much of the music bland. There was a deadly carol by Reger, a hybrid paean by Brahms and a French melody called "A Maiden Most Gentle" where the organ drowned out the women.

Works of Rorem and Poulenc added musical interest to the concert. Rorem's "Miracles of Christmas" cycle was lavish with good diction and rhythmic precision.Four motets by Poulenc, however, found the tenor section of the Cathedral Choral Society in painfully bad shape.

When it could be heard, the Choir of Boys and Men was awful. There were, however, several interesting if unintentional musical effects as various pitches were pitted against each other in the Old English counterpoint of "Welcome Yule."

The NSO Brass Quintet played a prelude and a postlude of Christmas music, as well as a selection from Bach's "The Art of the Fugue." Like most of the singing, the latter lacked accuracy and elegance.