Thousands of federal jobs will open up the first week in January, when many longtime workers who have been delaying their retirement in order to cash in on a special windfall Medicare benefit pull the plug and leave government.

Government agencies are expecting a flood tide of retirements between Jan. 1 and Jan. 8 of next year.

Beginning Jan. 1, all federal and postal employes will begin paying 1.3 percent of their gross salary (on amounts up to $35,700) for Medicare coverage. The Medicare tax does not apply to federal retirees.

In the case of federal workers who retire next month and have never paid for Medicare, the Health Care and Financing Administration (an arm of the Department of Health and Human Services), has ruled that they need only work one day in January to qualify for Medicare coverage at age 65.

Many federal workers have been delaying retirement pending the ruling on how long they would have to work in January to qualify for Medicare. Many have also been sweating out the calendar because they have excess annual leave (vacation time) over and above the 240 hours that employes are allowed to carry over from one leave year to another. They are getting a second break.

The 1983 leave year for most white-collar federal workers ends Jan. 8. Those who retire after that date will not be paid for their excess annual leave, which in some cases amounts to thousands of dollars.

But with the HCFA ruling on Medicare, they can have their cake and eat it--by retiring before Jan. 8 they will be eligible for Medicare and still get the benefit of their excess annual leave.

Normally about 7,000 federal workers retire each month. But the number of people retiring through September, October, November and this month is down. That is because so many were waiting for January and Medicare eligibility.

Now officials expect a lot of people to retire the first week in the new year. If those predictions hold true, there will be a lot of vacancies in government, though not all of them will be filled.

Understanding Medicare: Federal agencies late next week will get a fact sheet from HCFA explaining Medicare and the eligibility rules. Don't expect much help from your agency personnel office until that time. But remember, the work-one-day-in-January rule is official.

In mid-January, HCFA plans to issue a special pamphlet to federal and postal workers, telling them of the new Medicare tax and explaining what Medicare is.

All employes will have to pay the Medicare tax next year, even if they have worked in Social Security-covered jobs in the private sector for enough quarters to qualify for Social Security/Medicare benefits. That is because Social Security and Medicare are social insurance programs that require everyone to pay into them for as long as they work.