For months, the gala opening tonight of La Scala's 200th anniversary season has been touted as the most exciting event in this city of years.

But as it happened, many people here may remember yesterday nearly as well, for yesterday was the day of the great Dec. 6 blizzard - the day Italy's second-largest city was socked by a late autumn, storm that swept out of the nearby. Alps to tie up thorough-fares, nerves and opera logistics.

The airports were closed, throwing into doubt the arrival of VIPs for tonight's production of Verdi's "Don Carlo." It is an event made doubly fascinating by its remarkable cast, and by the attentions lavished on it by Claudio Abbado, the artistic director of the most famous opera company in a country where opera reigns supreme.

The snow fell for 14 hours, and by late yesterday La Scala officials were so uncertain of the status of the VIP lists that they would not say who was coming and who was not.

And even though the leading singers had been in town for days, some were balking at braving the storm, even from their hotels just across the street from Teatro all Scala.

The afternoon found Abbado, 44, in his baronial 200-year-old office, giving evidence that he is a man who remains ingratiating even under stress.

Thanks to the snow his guest was 40 minutes late, but thanks to Abbado's anniversary-night troubles, Abbado was 50 minutes late.

He sagged breathlessly into an 18th-century green velour arm chair and muttered, "You know, I love to ski. Maybe today I would have been better off doing that."

A singing rehearsal with three artists had been scheduled for 5 o'clock but so far there were no singers to rehearse.

Abbado gestured as if to call attention to the room which but for the huge black Petrof grand piano, might just as well be in Versailles. Gilt-framed mirrors reach to the high ceilings with marble and gilt armorists beneath them. The walls are gold silk brocade and the huge chadellier is crystal and gold.

"You know," he said with apparent awe, "this room is exactly as it was when it was Toscanini's," referring to the man who, next to Verdi himself, did the most to make La Scala a great opera house.

Abbado himself is anything but formal. He wore a tan cashmere turtle-neck with a brown jacket, and soon had his leg hanging over the arm of the antique chair.

Abbado was not relaxed for long, though. It was past 5, and his secretary brought news that the two leading ladies, Italian Mirelli, Freni and Russian Elena Obratzova, had asked not to have to come out into the cold to rehearse. Abbado took the news with seasoned equanimity. "Please try to get them both on the line for me."

In the meantime, he talked about opening the bicentennary season. "I decided two or three years ago it was either Verdi or a contemporary work." In Italy one need not look far to see how Verdi won out, for it is his face, and not a politician's, that appears on the Banca d'Italia thousand lire note, the closed equivalent to the one-dollar bill, and on the other side is a picture of La Scala. Both played central roles in the founding of the Italian state.

Abbado agreed that "Don Carlo" is perhaps Verdi's most eloquent comentary on the tyranny of the Hapsburgs, but it was also chosen for tonight, he says, because "it was perhaps Verdi's favorite of his works.

Abbado then bgan to tick off his plans for new productions in following seasons.

The phone sounded. "Here we go," Abbado said.

On the other end of the line was Obratzova, and Abbado talked with her in sweet, consoling tones. It was unclear at the end whether she was persuaded to risk her dark mezzo to the chilly elements.

Abbado was asked about the thoroughness of preparations for tonight's "Don Carlo." And the specifics of the answer explained better than might any suprlatives why La Scala is La Scala. "First, there were eight orchestral rehearsals. And since then there have been eight rehearsals on stage with singers, chorus and orchestra. Also, there are the numerous run-throughs like the one I am trying to get together here."

This, in today's opera, is as ambitious as a company can get. Ther recent Washington Opera productions of Mozart's "The Magic Flute" and Donizetti's "The Elixir of Love," for comparison, each had but one stage rehearsal with orchestra before opening nights. Within the next month and a half, La Scalas "Don Carlo" will begiven 10 repetitions, one to be televised, and probably to be shown on the American Public Broadcasting Service, says Abbado.

Another phone rang. "It's got to be Freni now," he said. It is, and the conversation is friendly but animated. From the tone of the conversation in Italian, Abbado seemed a little more comfortable putting pressure on his fellow Italian, the soprano, than on the Russian mezzo.

At about 5:30, a singer at last arrived. He was bass Nicoali Ghiarov, who sings the central role of Spain's King Philip II in "Don Carlo." Chiarov appeared voluntarily, and Abbado seemed appreciative.

Abbado now had worked to do - but not before confirming a rumor that he will direct the National Symphony for the first time in 1979. He has conducted in Washington before, including the majority of La Scala's Washington performances for the American bicentennial, but never the NSO.

"Rostropovich must have twisted your arm," it is observed. "Let's just say we work together," says Abbado. And he smiles and bids goodbye.

A few minutes later, behind the closed door of the artistic director's office, the unmistakable voice of the reluctant Freni can be heard soaring in its upper register.

But the snow is getting worse, and one does not wait to hear if Obratzova is there too.