The Supreme Court ruled 7 to 2 yesterday that an employer is not required to disrupt normal operations in order to let an employee observe a particular day of the week as the Sabbath.

The court held that the Civil Rights Act of 1964, as amended in 1972, does not compel an employer to put such an employee on a four-day week, pay someone else overtime pay to substitute for him on Saturdays, or breach the seniority plan incorporated in a union contract by arranging swaps of workdays.

In laying down the guidelines, the justices said that neither Congress nor the Equal Employment Opportunity Commision has spelled out how wmployers are to comply with the 1972 amendment ordering them "to reasonably accommodate to an employee's . . . religious observance or practice" short of "undue hardship."

Justice Byron R. White, in the opinion for the court, said that Trans World Airlines, Inc., and the International Association of Machinists & Aerospace Workers, AFL-CIO, had complied with the 1972 law in dealing with TWA employee Larry G. Hardison, who is a member of the Worldwide Church of God, which observes Saturday as the Sabbath.

For tht reason, White wrote, "we need not reach" the constitutional prohibition of laws "respecting an establishment of religion, or prohibiting the free exercise thereof.

TWA and the union had contended that the 1972 amendment both advances religion and entangles government in it. An employers' group, the Equal Employment Advisory Council, agreed. But 11 national Jewish organization and the General Conference of Seventh day Adventists denied this, saying that the amendment merely protects the religious freedom of persons who observe a day other than Sunday as the Holy Day of rest.

In the dissenting opinion, Justice Thurgood Marshall, joined by Justice William J. Brennan Jr., denounced the decision as "a fatal blow to all efforts" under he law to accommodate work requirements to religious practices.

Now, Marshall wrote, an employer "need not grant even the most minor special previlege to religious observers to enable them to follow their faith . . . This result is deeply troubling, for a society that truly values religious pluralism cannot compel adherents of minority religions to make the cruel choice of surrendering their religion or their job."

Sen. Jennings Randolph (D.W. Va.), who, as a Seventh day Baptist observes the Saturday Sabbath, introduces the 1972 amendment, which defines "religion" to include "all aspects of religious observance and practice, as well as belief."

Juctice White wrote that Randolph "made no attempt to define the precise circumstances under which the 'reasonable accommodation' requirement would be applied."

But, Juctice Marshall, terming the decision "intolerable" as a matter of law, said tha the court "adopts the very position that Congress expressly rejected in 1972, as if we were free to disregard congressional choices that a majority of this court thinks unwise."

He cited portions of the legislative history in which Randolph said the primary purpose of the amendment was to protect Saturday sabbatarians - such as himself - from employers who refuse "to hire or to continue in employment employees whose religious practices rigidly require them to abstain from work in the mature of hire on particular days."

The Senate approved the amednment unanimously. it and the House then approved a conference report containing it.

Hardison worked for TWA at its round-the-clock, 365-day-a-year maintenance base in Kansas City. He joined the Worldwide Church in 1968. This posed no problem as long as he was willing to work the night shift.

Tiring of that, he bid for and got a transfer to the day shif in another building with its own seniority list. He was at the bottom of it. To replace a vacationing employee, TWA asked him to work Saturdays. He refused TWA and the union tried but failed to develop acceptable solutions. Finally, he refused to report for work on Saturdays and was first.

Reversing the Eighth U.S. Circuit Court of Appeals, the Supreme Court held that TWA had made reasonable efforts to accommodate Hardison's religious beliefs, that the seniority system reasonably met the religious and secular needs of all TWA employees, and that the operation of a seniority system that is undistriminatory in design complies with the law even if discriminatory in effect.