Tomorrow, Americans will celebrate the 90th anniversary of Labor Day, a traditional day of rest that also unofficially marks the end of summer.

The first parade honoring working people was sponsored in New York City on Sept. 5, 1882, by the Knights of Labor, an organization that fought for workers' rights and dominated the U.S. labor movement from 1877 to 1887. In 1894, Congress declared the first Monday in September a national holiday. Since then, Americans in this country and in Canada have observed Labor Day with parades, picnics and political oratory. This year's U.S. events include:

*New York City. Generally the biggest and the brightest, Manhattan's parade will begin forming at 9 a.m. tomorrow, and the festivities are not due to end until about 5 p.m. The AFL-CIO expects more than 300,000 people to take part, either as marchers or viewers along the downtown route. The marchers will include construction workers with their heavy equipment, members of the Ladies Garment Workers Union wearing clothes they made for the occasion and members of Actors Equity and the Screen Actors Guild. There will be bands, floats and balloons, as well as appearances by stars of TV soap operas and a performance by the Radio City Music Hall Rockettes.

*Detroit. Tomorrow's float-filled downtown parade and rally will involve statewide union participation, including representatives of the building trades and teamsters. Marchers -- from waitresses to letter carriers -- will be in their working clothes, and political and economic labor themes will be echoed in speeches.

*Pittsburgh. Beginning with an 8 a.m. Mass tomorrow in St. Benedict the Moor Church near the Civic Arena, the observance will feature the first Labor Day parade held in the city in 20 years. More than 7,500 Allegheny County marchers -- teamsters, auto workers, mine workers, telephone workers, electrical workers -- will take part. The theme is "Put America Back to Work," and Lynn Williams, international president of the United Steelworkers of America, will be among the speakers at Gateway Center Plaza. The city is also sponsoring a jazz festival at Point State Park from 2 p.m. to dark.

*San Antonio. Representatives of AFL-CIO affiliated local offices around the state will attend a Samuel Gompers Breakfast tomorrow. American labor leader Gompers, who helped build the U.S. labor movement, died in that city in 1924; a statue of him stands in front of the Convention Center. There will be no parade this year, but Market Square is the scene of a musical Labor Day Weekend Fiesta that started yesterday and runs through tomorrow.

*Omaha. The Omaha metropolitan area's annual Septemberfest tomorrow will include a parade of trade unionists, a carnival, speeches and exhibits of local products set up by employes and employers.

*Small towns nationwide will celebrate the date with picnics and parades.