Wednesday, July 12 would be the perfect day not to take out-of-town visitors to the Washington Monument area for some relaxed sight-seeing. And if your Aunt Martha is eager to get off the regular tourist track and tour the U.S. Postal Service's L'Enlant Plaza headquarters, get her to wait awhile.

The reason for the warning is that the negotiations between the USPS and its 500,000 union members aret at the critical stage. The contract between the federal mail-moving corporation and its mail-movers expires July 20. And there is growing talk of a job action, or strike, unless things improve at the bargaining table.

So that the proper people get the point. AFL-CIO unions will call several thousands members in town July 12 with a message of militancy for the White House, Postal Service and American public. The monument and L'Enfant Plaza will be the site of some activities.

Some of this is normal labor-management rhetoric, the kind that occurs whenever any big contract is nearing deadline and each side wants to squeeze the other.

The big difference here is that it involves the biggest federal operation, the mail service. It touches nearly everybody and supplies the financial lifeline (through Social Security and tension checks) to 1 in every 5 Americans. This is the one situation where strike talk isn't taken lightly, even though strikes against Uncle Sam are illegal.

Postal workers are the shock troops of the bureaucracy. They've hit the bricks before - the last time bringing 20,000 members off the job even though the wildcat strike surprised, and was opposed by most national union leaders. President Nixon ended it with a 14 percent postal raise, and by assigning Army units to post offices.

This time, unions are pushing for a [WORD ILLEGIBLE] percent wage increase, a shorter contract, stepped up cost-of-living payments and promises that there will be no layoffs because of automation or or economy reasons.

The USPS, which recently jacked up first class mail rates from 13 to 15 cents a letter, has told union bargainers that it can't afford that kind of wage package. It demands the right to lay off or furlough workers whose pay and fringes account for 85 cents of entry dollar the USPS takes in.

The three unions involved in the major negotiations - American Postal Workers Union, National Association of Letter Carriers and Mail Handlers - have warned that they will stick together if members of any one union feel a strike is necessary. Privately the union leaders hope a strike can be avoided. But they are fully prepared, they say, for the walkout which - if successful - could be the biggest strike in American history.

USPS brass have been downplaying strike talk. But they have drawn up detailed contingency plans which envision the use of federal troops to pick up, guard and deliver mail if a general strike is called.

Emmet Andrews of the APWU, largest of the postal unions, says management is dragging its feet at the bagaining table an d concentrating discussions or "taking away" rights already in the current contract. J Joseph Vacca of the Letter Carriers says the talks are progressing unstatisfactorily. Mail Handlers chief Lonnie Johnson says his members says his members are upset with White House "interference" in the union-management talks. Administration officials have urged the postal unions to be "moderate" in their wage demands, pointing to the 5.5 percent pay "cap" President Carter plans for white collar civil servants.

The July 12 demonstration - including a march from the Monument to L'Enfant Plaza - will, in the words of one union chief, be "orderly and we hope informative. But if they don't get the message, it could be a very bad summer.

Federal Communications Commission workers have picked the National Treasury Employes Union to be their exclusive union bargaining agent. The aggressive, independent NTEU has been expanding outside the Treasury family, and now represent workers in the Department of Energy and Nuclear Regulatory commission.

Veterans Preference: The Senate and House committee working on the president civil service "reform" bill are on a collision course over the issued of a preferential hiring and tenure benefits for military veterans. President Carter wants to phase-out the lifetime preference, on grounds that it discriminates against women and minorities. The House Post Office-Civil Service Committee agrees with his stance.

On the Senate side, however, the Governmental Affairs Committee has voted against any change in veterans preference regulations within the federal service. Sen. Alan Cranston (D-Calif.), the Senate expert on veterans affairs says the Senate wil not allow any changes in veterans preference.

When the two versions of the form bill reach the conference committee stage - after first cleaing the Senate and House - the issue of what to do aout veterans preference and be a major striking point.