Throughout the US, it will be a good summer for opera

Opera used to be a winter pastime in major cities. Now it is a year-round affair, with the important companies fostering specific identities winter and summer.

But summer music is what this column is all about, and it is hard to know where to begin. This summer in particular is especially rich, because of visiting companies.

Specifically, the Vienna Volksoper is winding up its United States tour in Washington, D.C. (through May 20, at the Kennedy Center, on the shores of the Potomac), with performances of ''The Merry Widow,'' ''The Czardas Princess,'' and ''Die Fledermaus.'' The Metropolitan Opera finishes up its annual spring tour with visits to Minneapolis (through May 26), Detroit (May 28-June 2), and Toronto (June 4-9). The repertoire includes Wagner's ''Die Walkure,'' Puccini's ''Tosca,'' Britten's ''Peter Grimes,'' Mozart's ''The Abduction From the Seraglio,'' and this year's new productions - Handel's ''Rinaldo,'' Verdi's ''Ernani,'' and Zandonai's ''Francesca da Rimini.'' Since this tour commemorates the company's centennial, casting is on a high level, with the likes of Placido Domingo, Sherrill Milnes, Renata Scotto, Jon Vickers, Benita Valente, Marilyn Horne, Johanna Meier, and Samuel Ramey on tap.

The big news this season is the visit to Los Angeles of Britain's premier company - the Royal Opera House, Covent Garden - as part of the extraordinarily ambitious Olympic Arts Festival going on up to, around, and after the summer games. Sir Colin Davis leads all performances of three operas. The tour opens July 9 with a new production of Puccini's ''Turandot,'' being seen here before it is viewed in London. Gwyneth Jones assumes the title role, and Placido Domingo sings Calaf. Jon Vickers sings ''Peter Grimes'' in the controversial Elijah Moshinsky production of Britten's opera. Mozart's ''The Magic Flute'' completes the repertoire.

Another visitor to these shores, winding up its tour at the Met in late June, will be London's other major opera company, the English National Opera. The repertoire includes Britten's ''Gloriana'' and ''The Turn of the Screw,'' Prokofiev's ''War and Peace,'' Verdi's ''Rigoletto'' (in the controvertial updated version that puts the action in Little Italy, circa mid-20th century), and Gilbert and Sullivan's ''Patience.'' The tour includes Houston (May 24-27: ''Rigoletto'' and ''Patience''); Austin, Texas (May 31 & June 2: ''War and Peace''); the San Antonio Festival (the only domestic viewing of the company's ''Turn of the Screw,'' June 7 and 9, and ''Gloriana'' June 6 and 8); and finally New York, at the Met June 20-30 (omitting only ''Turn of the Screw'').

The premier summer company in America is now the Santa Fe Opera, in terms of choice of repertoire, in beauty of locale, and in the quality of the young artists it has discovered or launched over the years. A spectacular natural setting - the red adobe hills and crags of New Mexico - is fully exploited. The open-air house sits high on a hill, with the balcony covering sweeping up to the stars. In many productions, the backdrop is the shimmering night sky. And there is plenty to do in Santa Fe and environs during the beautiful sunny days.

This year, the repertoire is sure to please the opera connoisseur. The only ''staple'' opera is Mozart's ''The Magic Flute.'' Among the unusual works are a double bill of Korngold's ''Violanta'' and Zemlinsky's ''A Florentine Tragedy.'' A rare chance is offered to see a fully staged Strauss ''Intermezzo,'' with the noted Swedish soprano Elisabeth Soderstrom. The US premiere of Henze's 1976 opera ''We Come to the River'' boasts a libretto by the noted British dramatist Edward Bond. Finally, CiUarosa's ''Il matrimonio segreto'' completes the lineup. Performances begin June 30, but the full program of five operas does not come together until August. Closing night of the season is Aug. 25.

A company that has been giving Santa Fe a run for its money is the Opera Theatre of Saint Louis, performing at the outdoor Loretto-Hilton Theater in the middle of the city's glorious Forest Park. The ''novelty'' this year is a production designed by Louise Nevelson of Gluck's ''Orfeo and Eurydice.'' Other works to be heard include ''The Magic Flute,'' ''Madame Butterfly,'' and Britten's ''Paul Bunyan.'' The season runs from May 21 to June 24. The emphasis is on young singers, fresh productions, and an intimacy that cannot be duplicated at the larger opera houses around the country.

The Spoleto Festival of Two Worlds, Charleston, S.C., branch, has been establishing itself as a showcase for opera. The charm of the city itself is a strong calling card for the festival. This season, artistic director Gian Carlo Menotti has chosen Strauss's ''Ariadne auf Naxos,'' Lehar's ''The Merry Widow,'' and his own ''Juana la Loca.'' The operatic performances at the festival run from May 25 to June 10.

Wagnerites have never been as pleased as when the Seattle Opera Association started the Pacific Northwest Wagner Festival to mount its own ''Ring'' cycle. This season the company offers just one cycle, in German - a spruced-up version of its old production, with new staging by the celebrated Swedish tenor Ragnar Ulfung. It should be noted that in 1986 an entirely new ''Ring'' will be unveiled in the company's attractive home on the old World's Fair grounds.

Speaking of ''Ring'' cycles, the San Francisco Opera gets a new ''Siegfried'' this June, with the thrilling Hungarian soprano Eva Marton as Brunnhilde. This is a continuation of the project Begun last season with stimulating presentationS lf ''Das R eingold'' and'''Die Walkure.'' It will be completed next season with ''Gotterdamerung,'' when the entire cycle will be put together. Edo de Waart is the conductor, Nicholas Lehnhoff the provocative director, and John Conklin the imaginative designer. If Wagner is not for you but San Francisco in the early summer is, do not despair. The rest of the season at the War Memorial Auditorium includes ''Aida'' with Leontyne Price, ''Don Pasquale'' with lyric tenor Francisco Araiza, and ''Die Fledermaus'' with Peter Hofmann. That season runs through July 1.

The San Antonio Festival, mentioned earlier because it was affording the only domestic viewing of the English National Opera's production of ''The Turn of the Screw,'' offers not only opera but ballet, theater, and concerts. The Berlin Opera is bringing its staging of Bizet's ''Car omo Texas (May 19, 21, 23, 25). More important, the festival itself is giving the first US performance in over half a century of Rossini's final opera, ''William Tell'' (May 26, 29, June 1).

The festival is also mounting a production of Britten's ''Noyes Fludde'' in St. Mark's Episcopal Church (June 2 and 3). This festival is only in its second season, but already seems on the way to becoming an established fixture in America's spring and summer concert schedule. And it appears that the Berlin Opera and Ballet may have found a home away from home for a few weeks in May. Overall, it is a musically interesting festival, and there is the added bonus of the city itself - one of the most attractive downtown areas in any domestic metropolis.