IT IS THE first Monday of October, and the Supreme Court begins a new term today.

Once again there is speculation about possible changes in the court -- five justices will be 77 or older before the end of the year -- but they have consistently given evidence of remarkable good health, stamina and recuperative powers. Justice O'Connor is the only member to have joined the court since 1975, and there is no apparent reason to believe this stability will not continue this term.

About a third of the cases already scheduled for argument involve criminal law and procedure. The court will consider cases on the death penalty, search and seizure and the admissibility of confessions. Because most of these cases were won in lower courts by defendants, the American Civil Liberties Union has expressed concern that "the Court's eagerness to review them is particularly ominous." But no broad, landmark reversals are expected in this area. The court will also have a fair number of commercial and regulatory cases of the kind that are important to the economy but seldom exciting to the general public.

Most attention will be focused on cases interpreting federal statutes that have broad social consequences. Can Congress require schools to allow extracurricular religious meetings by student groups? Will the Baby Doe regulations on the medical treatment of handicapped newborns be sustained? Can white employees who have seniority under a union contract be laid off to protect the jobs of newly hired minorities? Will court settlements that make no provision for attorneys' fees in civil rights cases subvert the intent of Congress to encourage such litigation? Can a state's redistricting plan be invalidated as discriminatory even where black voting and registration have increased and blacks are being elected to office?

These are the issues that directly affect social institutions and strongly held beliefs. They involve subjects that Congress, the executive branch and the courts have been fighting about since the major legislative changes of the '60s. It will be a dramatic season.