Weep not for the 80-plus members of Congress who retired--either voluntarily or involuntarily--this month.

In a parting gesture of generosity to its retirees, the lame-duck session made a quick, quiet change in federal retirement rules. It allowed lame ducks to stay on the congressional payroll in January to get Medicare coverage and still have their pensions begin immediately. The background:

Less than a year ago, Congress decided to tighten civil service retirement rules. It changed the law that allowed annuities of feds (and members) to begin the day after retirement. (Bear in mind that very few members of Congress retired last year.)

The change said that a pension could not begin until the month after a worker retired, meaning that employes who wanted their annuities to begin, say in January 1983, would have to retire in December 1982.

But faced with a lot of congressional retirements this year, the House and Senate did an about-face. They loosened the rules they had tightened earlier, to benefit outgoing colleagues.

Earlier in the session, Congress voted to put federal workers (and members) under Medicare. U.S. employes began paying the 1.3 percent Medicare tax in January. The federal government said that employes who worked only one day in January would qualify for Medicare just as if they had worked in the private sector and paid full Social Security taxes for at least eight years.

In the closing days of Congress, several lame ducks decided that they wanted it both ways. They wanted Medicare coverage but did not want to have to play by the pension rules they had established earlier for regular government employes.

They wanted to be on the payroll in January and then retire with pensions beginning immediately the next day. But that was illegal. So what did the lawmakers do? They changed the law. They changed the law to say that anybody retiring during the first three days of a month--it just so happens that the new Congress started at noon Jan. 3--could still have his or her annuity begin immediately. None of this waiting a full month stuff.

Nobody on Capitol Hill knows (or will admit knowing) who first proposed the rules change. Suffice it to say that somebody did. And everybody went along with it.

The change cleared the House Dec. 14. It went through the Senate on Dec. 19. Senate-House conferees approved it Dec. 20. The president signed it--as part of the stopgap federal funding bill--Dec. 21.

Although the change would have benefited government employes--who thought they faced the same Medicare vs. immediate annuity choice--few of them took advantage of it, because they didn't know that Congress had changed the rule it had imposed just one year before.

Feds who wanted their annuity to begin in January retired in December, thereby missing out on Medicare coverage. Others who stayed until January to get Medicare were unaware that if they quit by the third of the month they could have had both Medicare and an immediate annuity.

Had they known what lame-duck members of Congress knew, most of the federal workers would have retired by Jan. 3 to get both immediate annuity and Medicare.

That change is official. It is contained in Public Law 97-377. But it came too late, and too quietly, to benefit most people retiring, unless they were retiring from Congress.