'Twas the night before the New Year and all through the house -- the Kennedy Center Opera House, that is -- there was a lot of monkey business.

It's been an American Ballet Theatre tradition here that New Year's Eve performances become the occasion for unannounced falderal and shenanigans. This year, the troupe scheduled in advance a "45th Anniversary Celebration" (ABT was founded in 1939) featuring "festive surprises." "Swan Lake, Act II," not in current ABT repertory, was to lead off, followed by a group of "surprise" pas de deux, and then "Graduation Ball," the revived David Lichine ballet comedy about a girls' school in old Vienna.

An insert in the printed program revealed a bit more, including the titles of the pas de deux -- but that turned out to be only the beginning of the surprises.

For instance, the insert indicated the "Don Quixote" pas de deux, with the cast marked "Marianna Tcherkassky and/or Cheryl Yeager" with "Kevin McKenzie and/or Danilo Radojevic." What we got was a pas de quatre with all four. It was a few bars into the adagio, with Radojevic and Yeager doing the honors, when Yeager slipped out and Tcherkassky took her place; a little further along, and McKenzie nudged out Radojevic. Eventually the whole quartet got into the act, with some additional gags thrown in for good measure -- as when the two ballerinas headed onto a collision course with their pas de cheval diagonals. It was a barn-burning performance all around, on virtuosic grounds.

The "Swan Lake" excerpt had Cynthia Gregory as Odette and Clark Tippet (replacing injured Ross Stretton) as Prince Siegfried, in a performance of masterly pathos and rapture. But the classic wasn't left untouched by surprise either. As the curtain opened on the misty lakeside, the figure of Von Rothbart, the evil magician (Michael Owen) rose mysteriously from the floor at center stage. In the "Cygnets" section, the four arm-linked dancers made their customary entrance, but then defected one by one to the wings, until a lone cygnet was left, giving a final "oh, what the hell" flip of the hand at the cadence.

The first surprise duet brought Cynthia Harvey and Gil Boggs out for Balanchine's "Tchaikovsky Pas De Deux," in a brilliant, no-nonsense performance -- the surprise, or rather, the astonishment, lay in things like Boggs' sky-high cabrioles and Harvey's flinty pyrotechnics. Next up was "Odalisque," a solo to music by Satie by Glen Tetley -- here the surprise was that Martine van Hamel, the most patrician of ABT ballerinas, could make something of this nothing, holding the stage with her sultry stretches and insinuating poses, despite the anemia of the material.

Then came the "Don Quixote" foursome, and last of all, "Graduation Ball," which had more surprises per minute than all the rest of the program in toto.

For starters, the travesty role of the Headmistress was taken by ABT ballet master and former dancer Terry Orr, and the General was portrayed by recently appointed ABT associate director John Taras (who performed the same part in the Ballet Theatre premiere of the work in 1944). They made a deliciously comic couple -- Taras was so bowed over backward with self-satisfied pomp that he seemed to be advancing and retreating at the same time. Washington Opera singers Cyndia Seiden and Jerry Hadley took the stage with interpolated arias from "Tales of Hoffmann" and Lehar's "Land of Smiles." Amanda McKerrow and Robert La Fosse intruded with a more or less traditional and extremely grand "Grand Pas de Deux" from "Nutcracker." The competition dance, which calls for rival ballerinas, was danced instead by Johan Renvall and Peter Fonseca, complete with whirling fouettes and a wonderful barrage of strictly masculine bravura. Then the cadets launched into the Toy Soldier's dance from Mikhail Baryshnikov's version of "Nutcracker," only to be joined by the Cat from the same choreographer's "Cinderella," who ended up playing his own cat-and-mouse game by aping the Mouse King antics in "Nutcracker."

One knew the jig was up when toward the end of all this, the strains of "Auld Lang Syne" began to make themselves heard in the orchestra simultaneously with the Johann Strauss tunes of "Graduation Ball." The cast paraded on with noisemakers, and just before the curtain fell, a banner was lowered from the flies with the inscription, "HAPPY NEW YEAR." And the very same to you.