Thousands of federal civil servants who have been legally enjoying three-day weekends for years will have to return to the Monday through Friday, 9-to-5 grind shortly unless the Senate makes permanent the so-called flexitime program. It is scheduled to expire July 23.

The White House backs the flexitime program and the House approved it weeks ago. Legislation that has been awaiting Senate action may come up for a vote today. It can't come too soon for the hundreds of thousands of U.S. workers who have grown accustomed to flexitime.

About one in every five of Washington's 350,000 feds is now working some kind of flexible shift.

Uncle Sam has been experimenting with various flexitime programs for the last six years. Flexitime allows some workers to beat the a.m. and p.m. rush hour by coming in early, working eight hours and leaving early. It allows others to split their eight-hour work day to attend midday classes. One form of flexitime also makes it possible for employes to work 10-hour shifts four days a week. The current program has 11 days to run.

Unless the program is extended, employes who are on the four-day week will have to shift back to eight-hour days, five days a week. The flexitime experiment allows the government and employes to waive mandatory payment of overtime if a work day runs longer than eight hours. Four-day weeks would have to end if the program is not extended, but most other flexitime schedules could continue because no overtime is involved.

Flexitime, when managed properly, allows agencies to be open longer to serve the public and virtually eliminates tardiness.

Lately, most of the energy and attention devoted to federal personnel matters have centered on Senate-House efforts to reach a budget compromise. The flexitime measure appeared to have fallen through the cracks. But the Senate apparently now is ready to give quick approval to the bill, which has bipartisan support and is one of those rare civil service items that will make life a little more pleasant for the people who work for the government as well as the people served by it.