Yes, if you're a federal employe, you've got to go to work today.

Even though at 12:01 a.m. Saturday the government lost legal authority to perform most of its normal functions, and even though the deadlock between the president and Congress could continue, federal workers are expected to be on the job today.

But unpaid furloughs could begin, and many government operations could stop, as early as tomorrow if the impasse is not resolved. The White House said last night that furloughs could begin Tuesday, and that 400,000 civilian workers could be laid off by Thursday.

Some federal employes might be asked to work as "volunteers," a spokesman suggested, with pay to be made up after a funding bill is passed. This happened during a more limited funding lapse in 1979.

Military personnel and civilians working in national security, health care, tax collection, and federal prisons would be kept on the job no matter what, officials said.

The mail will probably continue to arrive--the Postal Service gets only 2 percent of its money from the treasury--and Social Security checks, financed through a trust fund, will still be written. But many government offices, parks and museums would be closed starting tomorrow if no agreements were reached.

Congress has missed funding deadlines for parts of the government before, and it has occasionally let funding run out on purpose for specific agencies. But Office of Management and Budget spokesman Edwin Dale said this is the first time that nearly all agencies of the government have been left without money for any extended period.

Nonethless, under a double-edged interpretation of the rules issued by Office of Personnel Management Director Donald J. Devine, bureacrats were to come to work today no matter how the budget battle turned out.

If Congress passes and President Reagan signs a new funding resolution by the start of work today, employes will simply carry out their regular jobs. But if no funding resolution has been passed, Devine said, the federal work force will still be needed to carry out an "essential function"--shutting down the government.

"The shutdown would take two or three days to complete," Devine said. "All of the actions related to closing down the government fall under an essential function."

And since existing laws permit the government to move ahead with "essential functions" even when there is no authorizing legislation in place, all government employes can and should continue to come to work, he said.

After the shutdown is another question, but no one yesterday expected the funding standoff to last that long.

In recent years a race against the funding clock has become a regular feature of Congress' budget battles. And political disputes have led to previous lapses in funding authority for specific agencies. For example, the Federal Trade Commission closed for a few days in March, 1980, while Congress battled over its authorizing statute.