Uncle Sam's power to charge federal workers -- including 75,000 here -- to park at the office expires at midnight unless the U.S. Court of Appeals acts today to extend the life of the controversial pay parking program.

That means that unless the court grants the government a stay most federal workers -- who now pay from $12 to $40 per month for a parking space -- will be able to park free Monday, if they have the proper permit.

Many U.S. workers -- about two of every ten here -- have been paying to park since late 1979. President Carter ordered agencies, from Washington to Honolulu, to begin charging employes one-half of the local commercial rate for a space at the office. Under the Carter program, federal workers would begin paying full fees -- as high as $80 per month in some cases -- later this year.

The American Federation of Government Employees and other unions have been fighting the pay-parking order almost from the day it was issued by the Office of Management and Budget in 1979.

Last week, U.S. District Court Judge Harold H. Greene issued a summary judgement ordering the government to stop pay parking. Greene had ruled earlier that the method the government used to implement pay parking for energy-conservation purposes -- an OMB directive rather than congressional action -- was improper.

The government asked Greene to grant a stay that would permit pay parking to continue pending a decision to appeal to a higher court. Greene said that such an appeal could take up to a year. Last Friday he told the government he would give it five days -- the time is up at midnight -- to see if it could get the U.S. Court of Appeals to grant a stay.

If the government wins a stay, it will be bale to continue to collect parking fees at least temporarily. If the government is denied a stay, federal workers, as early as Monday morning, could be freed from the OMB-directed pay parking program -- that is if they can find a space.

The government, obviously wants to keep the pay-parking program which would bring in an estimated $31 to $47 million each year with full fees charged. It is also anxious to have the current program upheld. Otherwise, it could be ordered to make refunds -- running into the millions of dollars -- to more than 300,000 workers who have been paying to park since 1979.