To announce the discovery of a major new work by Claudio Monteverdi is like saying that a complete, authentic Tenth Symphony by Beethoven has turned up. The parallel is precise, for Monteverdi in his time (1567-1643) lived through the closing of one era in music and well into the opening of its successor, just as Beethoven (1770-1827) did.

The world is so much more familiar with the music of Beethoven (whose 209th birghday it is marking today with all kinds of hoopla) than it is with the music of Monteverdi, that it probably does not -- perhaps cannot -- ever realize that the two men were certainly equals in genius.

Washington will have an opportunity of hearing this most important discovery in Monteverdiana on Friday and Saturday, Dec. 21 and 22, when Denis Stevens conducts the U.S. premiere of the Christmas Vespers in the Shrine of the Immaculate Conception. Stevens, who is one of the world's foremost Monteverdi scholars, gave the world premiere of the newly published music last September in St. John's Church in London. He arrives from London today to prepare this week's performances.

Stevens, whose edition of the complete letters of Monteverdi will be published next year -- the first time the entire known collection has been printed -- is very precise about the Christmas Vespers: "What we have is a synthesis as Monteverdi might have known it," he says of his work as an editor.

The key to the whole thing is the fact that Monteverdi, during the 30 years in which he was in charge of the music at St. Mark's Cathedral in Venice, wrote new music every year for the Christmas festivities in that great church. In a letter dated Feb. 2, 1634, Monteverdi wrote, "I have received two letter from Your Reverence. One arrived before Christmas, at a time when I was very busy writing the midnight mass, which, according to the traditions of this city, is a mass composed afresh each year by the maestro di cappella."

What has puzzled music scholars for years was where all this music "composed afresh each year" had gone. For a long time it was simply presumed that it had been lost or destroyed like a hundred cantatas by Bach and countless other works that are known to have existed at one time.

That 1634 letter was not the only one in which Monteverdi had talked about his music for Christmas. Eighteen years earlier he had written to his friend, Alesandro Striggio sayng, "I am slow in writing because of my work on the mass for Christmas Eve: I have had to spend the whole of December, almost without a break, composing and copying it. Now, praise be, I am free . . . since all my work for Christmas Eve and Christmas Day is behind me and I have for a while nothing to do for St. Mark's. I . . . will even accept any new commission from Your Excellency."

Obviously a substantial body of music for this season of the year did at one time exist. Stevens explains his detective work this way:

"I suggest that a large part of this music appeared in the two collections of 1641 and 1650, where considerable numbers of apparently disparate and unrelated compositions have hitherto defied attempts to group them into acceptable liturgical entities on a more sweeping scale." Citing the opinion of Italian scholar Mons. Biella of Milan that taken together the various collections of Monteverdi music contain all of the required music for Vespers . . . "I urged that the combined publications be used to reconstruct Vespers for six major feasts from the Temporale and seven more from the Proper of the Saints."

Then, adding his profound knowledge of performance practice in Monteverdi's music to his solid musicological scholarship, Stevens decided on the resources that would be appropriate for so important a liturgical service as Vespers at Christmas time, which is one of the greatest feasts in the church year. Thus his new edition calls for much the same resource as the famous Marian Vespers of 1610 in which Monteverdi used seven soloists, double choir, and orchestra of strings, trombones and organ.

When he reached the "Magnificat, which closes the Vespers, Stevens made the corrections and additions that were necessary to repair damage caused long ago when the music was printed incorrectly. Some inkling of the public's awareness of an appreciation for the new music can be duduced from the fact that the Vespers of 1610 have been recorded more frequently than any other music written before the middle of the 17th century.

In the Shrine on Friday and Saturday, Stevens will have at his disposal the National Shrine choir, soloists, and orchestra as called for in his score. The biggest variable between Monteverdi's performances in St. Mark's (aside from the unknown differences in the way voices and instruments sounded then compared to the way they sound today) is the fact that St. Mark's is a church of moderate size in the shape of a Greek cross, with arms of equal length. The Shrine on the other hand, is one of the largest churches in the world, with a flair for reverberation that swallows up huge quanities of sound. Can Monteverdi's agile lines and choral blocks emerge clearly in the vast distances in the Shrine?