FOR ALMOST five years, the federal courts and prosecutors have known that the provisions of the Speedy Trial Act would become fully effective on July 1, 1979. Next Sunday is July 1, 1979, and guess what? The courts and the prosecutors aren't ready.

So there will be either a flurry of activity on Capitol Hill this week or one in the courtrooms next month. The provisions involved say that the government must bring criminal defendants to trial within 100 days after they are arrested. If it doesn't do so, the charges against them are to be dismissed.

For many months now, the Department of Justice and the Judicial Conference, which makes policy for the courts have been agitating for a change in that time limit. They want it to be 180 days because they contend, 100 days is not enough time to get federal cases ready for trial.

The Senate Judiciary Committee has already turned that plea down, noting that in the 12 months ending a year ago 80 percent of the federal cases had been terminated within 100 days. Instead, it proposed that the deadline be put off until 1981. That would give the courts and the prosecutors more time to either adapt to the law or come up with better reasons than they now have for wanting it changed.

As distasteful as the idea of postponing the effective date of any law is, especially one that has been waiting in the wings for five years, Congress doesn't have much choice. If next Sunday's deadline is not changed, hundreds of federal prosecutions may be dropped. The Senate approved the extension last week and the House of Representatives should act on it this week.

That, of course, will be giving the judges and prosecutors at least part of what they want. One reason they are not ready - perhaps the principal one - is that many of them think the Speedy Trial Act is a bad law. The best way to get Congress to repeal it, some figured, was to enforce it strictly and dismiss criminal cases wholesale. Thus, why bother to get ready to follow its limitations?

The act was a good idea when former senator Sam J. Ervin introduced it. It is a good idea now. It was needed then; and it is needed now because cases drag on and on without rhyme or reason. Occasionally, the government or a defendant does require more than 100 days to prepare for trial, but the act provides for exceptions where they are needed.

Indeed, the act's basic weakness may only be in the sanction it imposes. If the law could provide that prosecutors and judges were threatened with a loss of pay, rather than the dismissal of a case, at the end of the 100 days, the odds are they would be ready now for what is supposed to happen next Sunday.