This weekend will be critical for the nation's 550,000 postal workers, their unions and managers, and for the American people who shove 293 millions pieces of mail into the postal system every day.

The on-again-off-again prospect of a postal strike may be on again Saturday. That is when Harvard professor James J. Healy announces the success, or failure, of 15 days of talks between postal officials and postal unions. Under the agreement, Healy has until Friday to bring the two parties together - acting as a mediator - or to turn into an arbitrator and dictate terms of the biggest contract of the year.

The two issues on the table are money and job security. The rejected money offer amounted to 19.5 percent over three years. The contract also included a promise that no workers would be laid off during the life of the agreement.

Union members rejected that contract earlier, leading to the mediation and arbitration process.

Unless the unions and the postal service can agree to a compromise by Friday, Healy will take over. and on Saturday, tell them what they have to do. It is at that point that militant union leaders - mainly from the New York City area - could trigger a strike, notwithstanding various laws and court orders barring strikes.

Bob Williams, postal expert for the Federal Times, is predicting that Healy will come up with a face-saver. It would allow both sides to take something important back to their constitutents.

Williams believe Healy may grant the unions more money - something the UPS has refused to do - either in a bigger wage settlement or in a shifting of funds so that the largest increases would come early in the contract. At the same time, Williams feels the no-layoffs clause may be modified by Healy, perhaps to "grandfather present postal employes into lifetime tenure and to allow the U.S. Postal Service to lay off new workers when automation or changing mail volume make their jobs obsolete.

There is talk that one or more of the unions may send Healy's arbitration award - whatever it is - back to the membership to ratify or reject. But that isn't the way the system is supposed to work. If Healy switches form mediator to arbitrator, he is supposed to have the final say. Whether militant union leaders, who want a strike for both economic and political reasons, will but that remains to be seen.

Survivor Benefits: Federal retirees who marry after Oct. 1 will have to say it in writing, if they want their new spouses to qualify for survivor annuity. A decision to provide for the annuity would reduce the monthly pension of the retiree while he or she lives. It is now done automatically when a retiree marries, but after Oct. 1 the retiree must make the choice. It is something would-be spouses of federal employes ought to be aware of.

Telephone Communications Specialist: HEW has a Grade 12 job, deadline for applying today, for an applicant with civil service status. Call 443-5460.

John Golden, personal director for the Commerce Department, is taking a one-year sabbatical at Havard. Joseph Brown, his deputy, has taken over as acting director.

Hatch Act: President Carter has promised to push for Senate action on legislation already by the House to allow federal and postal workers to get involved in partisan political campaigns.

The Hatch Act modification bill, proposed by Rep. William Clay (D-Mo.), has been quietly gathering dust in the Senate. Clay threatened to hold up House action on the President's civil service reform package until somebody at the White House said something nice - and positive - about the Hatch Act changes. Carter has sent Clay a letter pledging to remind the Senate that the bill is in their hands and that he favors its passage.