At night and on weekends, someone is "on duty" for every major department of every government agency.

Most of the time, "O.D." is a yawner. You merely stay by the telephone in case something big breaks. You don't have to go into the office. All you have to do is postpone going to the movies and avoid having one martini too many.

But Jack Lichtenstein of the Commerce Department's public affairs office got a live one a couple of weeks ago when he had "duty" on a Saturday night.

It began with a midnight phone call from Jerry Willis, who was working the switchboard at Commerce headquarters downtown. Pan American World Airways had just called. A man named Mile Mokhalane was standing at its counter at rapidly emptying National Airport.

He was from Lesotho, where he works as a statistician for the government. He spoke very little English. He knew no one in Washington. The one thing he did know was that he was in the United States to attend a training program sponsored by the Commerce Department's Bureau of Economic Analysis.

Mokhalane had told Pan Am's desk personnel that he thought someone from Commerce was supposed to meet him. But no one had, and it was becoming increasingly obvious that no one would. The man had never been in the U.S. before. He was getting scared.

What would you do, duty officers?

Lichtenstein immediately called Mokhalane through Pan Am. He explained that the situation probably couldn't be straightened out until Monday morning. In the meantime, he would try to get the man from Lesotho a place to stay.

Traveler's Aid? Closed. The Lesotho embassy? No answer. Finally, Lichtenstein determined that Mokhalane had enough U.S. currency to cover two nights in a motel. So he booked Mokhalane into the Hospitality House in Arlington, just a short drive from the airport.

The next morning, Lichtenstein called the motel to check on Mokhalane. Doing fine, the statistician reported. And on Monday morning, Mokhalane caught up with his training program.

Why the confusion? According to Alice Swerdling, chief of reception information services for the Washington International Center, instructions had been mailed to Mokhalane. "He just might not have received them," Swerdling said.

In any case, all's well that ends well. And any agency that's thinking of phasing out its "duty" system ought to pass around photocopies of this column. Once in a while, "duty" officers and the "duty" system produce excellent results.