Air fares. At summer's end for the past two years, U.S. airlines flying the Atlantic have offered special winter air fares, good for travel from about mid-October to mid-March. Unfortunately, the deadline for purchasing most of these bargain tickets was Sept. 30. Keep an eye out for new ads in case the carriers decide to repeat the offer again later this year.

- Accommodations. Many hotels throughout Europe reduce rates from 10 to 50 percent during the off season, according to the 24-nation European Travel Commission. As in the United States, many of the best rates are on weekends in big cities when demand from business travelers is light.Among the reductions: 20 to 30 percent off hotel rates in Belgium during January and February; up to 20 percent off in Greece in winter; up to 20 percent off in Amsterdam in winter; lower weekend rates in France and Denmark; up to 20 percent off in Portugal winter and spring; and 30 to 50 percent off in Yugoslavia from November to April.

- Coming events. The winter cultural calendar, compiled by the European Travel Commission, is full. Among the big events:

Paris inaugurates the first full season of its new opera house, Opera de la Bastille, on Nov. 13 with a production of "Otello," starring Placido Domingo.

Salzburg marks the 200th anniversary of the death of Mozart with a production of his opera "The Magic Flute" as well as a series of concerts, recitals and chamber music performances, Jan. 25 to Feb. 2.

The Royal Danish Ballet in Copenhagen has scheduled three world premieres during its winter season.

The Royal Academy in London is exhibiting a large collection of paintings by Monet through Dec. 9.

Luxembourg plays host to an international exhibit of antiques and fine arts March 7 to 10.

The Danish Museum of Decorative Art in Copenhagen is featuring a centennial exhibit, "Flowers for Every Rhyme and Reason," through Dec. 2.

For a full winter calendar of events, send a stamped, self-addressed envelope (long size) to European Events, Box 1754, New York, N.Y. 10185.