THE VISIT of the Metropolitan Opera to Washington beginning tomorrow is no routine affair. The company appeared at the Kennedy Center for the first time last spring for a single week. This year its visit has been stretched to two weeks, the longest stay of its tour.

James Levine, the company's music director and the strongest of its conductors, will be on the podium for eight of the 14 performances -- a further mark of the importance Met officials attach to its appearances here. The other conductors will all be new to Washington: David Stivender, the company's superlative chorus master; Thomas Fulton, and Neeme Jarvi. There is particular interest in Jarvi's appearance, since, during the second Met week, Jarvi also will be appearing as guest conductor of the National Symphony Orchestra, replacing the late Kiril Kondrashin who died suddenly a few weeks ago.

A result of the Met's longer stay here is that, with twice as many seats for sale, a number are still available for most performances. As of a few days ago, 10 out of the 14 were not sold out.

A recent announcement from the Met brought the word that Sherrill Milnes will sing the title role in the first of the two performances of Mozart's "Don Giovanni," scheduled for the April 25 matinee. James Morris, who was to have sung both Dons, was recently hit by a flu bug and will not sing the role until the second performance May 2.

The Met is bringing seven operas to Washington plus the Verdi Requiem, and is giving each bill twice. Of the seven, six are, like the Requiem, extremely familiar: La Traviata, Manon Lescaut, Don Giovanni, Cavalleria Rusticana, Pagliacci, and Samson and Delilah. The one novelty is Kurt Weill's "Rise and Fall of the City of Mahagonny." Even this is not unknown in Washington. It was performed by the Opera Society some seasons back in a production remembered for vivid direction at the hands of Ian Strasfogel, whose father was for many years one of the principala coaches at the Metropolitan.

"Mahagonny," as the opera is generally known, is something of a cult piece today. There are those who race over the cynicism of Berthold Brecht's book, which portrays Mahagonny as a city totally dedicated to material pleasures. The unspecified mythical locale is in the southern United States, which is why one of the big (and famed) numbers in the opera is called "Moon of Alabama."

In the second act there are highly pictorial enactments of Gluttony, Love, Fighting and Drinking, each carried to excess. Murder and seduction are greatly admired in the cit of Mahagonny where inability to pay one's just debts is the greatest of crimes.

Like several highly regarded 20th-century operas, "Mahagonny" revels in the sordid. What "Mahagonny" has not achieved up to this time, however, is the acclaim on musical grounds that has gone to Alban Berg's "Wozzeck" and "Lulu." Those are widely held to be 20th-century masterpieces. Certainly the Met's production of "Lulu" this season, which is one of the finest accomplishments in the entire history of the company, makes inarguably clear the overwhelming genius of that work.

It is a great pity that matters of scheduling did not permit the Met to bring "Lulu" to the Kennedy Center. Nonetheless, there will be a great deal of interest in seeing its "Mahagonny," which has won both praise and adverse criticism.