By 7:30 p.m. last Friday, most newspaper and wire-service reporters in town had flipped their notebooks shut and gone home for the weekend. The network news broadcasts had just flickered off.

It was, in other words, the perfect time for the Justice Department to put out a bad-news story.

In Los Angeles -- where it was 4:30 p.m., moments before U.S. District Court would close for the day -- Justice Department lawyers went into the chambers of Judge Ferdinand F. Fernandez to say they were dropping fraud charges against General Dynamics Corp. and former NASA chief James M. Beggs.

The story that the Justice Department had botched one its biggest defense-fraud cases missed the early editions of The Washington Post and The New York Times. It never passed the lips of Dan Rather or Tom Brokaw, which means it just didn't happen for millions of Americans. And the sketchy accounts that made it into print appeared on Saturday, a day when many newspaper readers are otherwise engaged.

Justice Department spokesmen here deny that they tried to bury the General Dynamics story, saying they had planned to issue a press release Monday and were surprised by the late-Friday move of U.S. Attorney Robert C. Bonner in Los Angeles.

But whether the timing was premeditated or fortuitous, it continued a pattern in which the Justice Department has waited until week's end to put out unfavorable or unflattering news. The practice probably dates to Plato's Republic, but the Reagan administration has set a new standard.

As dusk approaches on Friday, reporters on deadline have little time to sift through legal documents or seek critical reaction. Offices are closing; messenger services are slow. This often means a story that reflects the administration's viewpoint.

The textbook classic remains the Justice Department's January 1982 announcement, in the Bob Jones University case, that it would no longer challenge tax breaks for schools that discriminate against blacks. The word was put out around 6 p.m. on the same Friday that Justice had settled major antitrust cases against AT&T and IBM.

At the Treasury Department, "They waited until the press left the pressroom and the lights were out," a former official recalled.

The strategy was dictated in a now-famous memo from Ann Dore McLaughlin, then assistant treasury secretary for public affairs. She said the timing "insures the first wire stories out -- and thus the most widely used, especially by the broadcast media -- will contain our rationale."

Justice carried on this tradition in December, when it waited until 5:45 p.m. on a Friday to announce that Attorney General Edwin Meese III had decided not to seek an independent counsel to investigate U.S. Ambassador to Switzerland Faith Ryan Whittlesey. On a Friday evening the next month, Justice disclosed that it had asked for a court-appointed prosecutor to investigate former White House aide Lyn Nofziger in the Wedtech case.

Since Meese became entangled in the Wedtech case, reporters have been eagerly awaiting his latest financial disclosure form. After obtaining two extensions, Meese signed the form early last week before leaving on a trip to Vienna, but Justice said it would not be released until the official deadline -- Friday. (As it turned out, officials withheld the form to review it further.)

The indictments involving General Dynamics' Divad antiaircraft gun had been announced by Bonner at a televised news conference. But when the case was dropped, with Justice admitting its theory of prosecution was fatally flawed, the news was posted on a bulletin board in the Los Angeles court.

Bonner, according to a spokesman, completed the paper work early and agreed to a Friday filing after defense lawyers from other cities said they wanted to avoid flying back to Los Angeles.

Justice spokesman Patrick Korten said the announcement "was not supposed to go out on Friday. I didn't know about it until 9 o'clock when reporters started calling me at home.

"I was furious," Korten said. "We wound up with a weekend full of stories where I had no opportunity to explain what we had done."

On Monday, Assistant Attorney General William F. Weld invited reporters to his office, where the lanky prosecutor answered questions for 70 minutes on what he knew would be a no-win story.

But Beggs, whose December 1985 indictment had been the lead story in most American newspapers, remained skeptical about the late Friday maneuver.

"That was calculated, you know," Beggs said. "They waited till 4:30 California time, which is the last hour they could get to the judge . . . . We knew about this early Friday morning, but we couldn't say anything because we were under a gag order."