Labor Day for hundreds of thousands of union workers will be Solidarity Day III in Washington and 150 cities across the United States - a day of parades, picnics, and political attacks on the Reagan administration.
Unions who participated in the Aug. 28 mass march in Washington are set for new rallies against Reaganomics. Says William W. Winpisinger, president of the International Association of Machinists and Aerospace Engineers, ''We do not believe President Reagan's fables that the nation is enjoying 'economic recovery' and that everybody is better off.''
This year, AFL-CIO's Executive Council decided that - for more political effectiveness - Solidarity Day rallies should be spread out across the country with each of 35 council members speaking at rallies in his or her hometown or union area.
For Mr. Winpisinger, this will mean three appearances - in Fort Lauderdale, Fla., in early morning; then in Des Moines, Iowa; and finally in Alameda County, Calif. The personal appearances of council members and Lane Kirkland and Thomas R. Donahue, the president and the secretary-treasurer of AFL-CIO, are planned as a strategic support for big decisions looming for the federation in early October.
AFL-CIO's Executive Council wants a preprimary endorsement of a candidate for the Democratic nomination for presidency, a step that it considers ''in the best interests of America's working men and women'' and one necessary to ''ensure the solidarity that is essential for the labor movement'' if it is to have an effective voice in the political process.
The federation's general board of union presidents will meet on Oct. 1 to ratify or reject the plan to endorse a single candidate before state primaries. If the still controversial plan is approved, the biennial convention in Hollywood, Fla., opening Oct. 3, will take the final steps. Generally, the earlier approval of a single candidate is considered likely to help the candidacy of Walter F. Mondale and to undermine growing labor support for Sen. John Glenn of Ohio.
Labor's Solidarity Day I was a massive rally in the Washington Mall two years ago. Solidarity Day II, in the fall of 1982, was designated as ''a march to the polls.''