The New York City Opera and the union representing its striking musicians tonight reached a tentative settlement in the eight-week-old labor dispute that has caused the cancellation of the opera company's entire summer season.

Details of the proposed agreement were unavailable pending a ratification meeting by the 69 members of the orchestra, which is scheduled for 4:30 p.m. Monday.

The opera company said tonight it anticipated resuming performances on Sept. 21--eleven weeks after its scheduled July 7 opening. Only eight of its originally programmed 19 weeks of opera will be presented under the terms of the apparent settlement.

A spokesman for New York Local 802 of the American Federation of Musicians said the union has been informed that rehearsals are to resume next week, if the accord is ratified.

NYCO chief negotiator Martin J. Oppenheimer said of the tentative settlement, "We hope it's the beginning of a new era. Everybody got a little and everybody gave up a little."

Sources said the musicians received salary increases and guarantees of additional work weeks in future seasons.

The strike, which began July 6, has centered on the key issues of a City Opera Company-mandated reduction in the musicians' number of annual work weeks, as well as the musicians' demands for wage increases and a reduction in workload.

The strike has been particularly acrimonious, with NYCO director Beverly Sills calling the musicians' demands "off the wall" and the musicians union charging Sills with "mismanagement" of the opera company.