When Congress meets late into the night or on weekends, about the best anyone can hope for is simple mischief. It's a safe bet that whatever lawmakers do won't be on the subject at hand.

That's exactly what happened Saturday when the Senate held that cliche, the "rare Saturday session." The subject was supposed to be major trade legislation. Instead, the Senate found itself in a battle over a balanced budget, something that has been a fiction for many years.

The Senate passed 59 to 11 an amendment to the trade bill that would require President Reagan to do something by Sept. 15 that he hasn't done in more than six years in office: submit a balanced budget. The amendment, by Sen. Phil Gramm (R-Tex.) of Gramm-Rudman-Hollings fame, also would require Congress to act on a balanced-budget constitutional amendment by Oct. 1.

Lest anyone think the action was definitive, Senate Majority Leader Robert C. Byrd (D-W.Va.) introduced an amendment to prohibit the Senate from voting on a balanced-budget constitutional amendment until Reagan has sent a balanced budget to Congress, and until the Senate passes a campaign-finance revision bill that Byrd considers a top priority but against which Republicans have successfully filibustered. Of course, the trade bill would have to pass and be signed by the president.

And even if the amendment(s) survive the legislative process, keep in mind that Congress has already broken a much tougher law, Gramm-Rudman-Hollings.