President Reagan celebrated Thanksgiving with family members yesterday at his 688-acre ranch in the mountains north of here.

The president and Nancy Reagan went horseback riding this morning before welcoming their two daughters and the president's brother Neil for a 5 p.m. Thanksgiving dinner of turkey, cornbread dressing, gravy, string beans, sweet potatoes with marshmallows, cranberry mold, olives, carrots, celery strips, mincemeat pie with brandy sauce and persimmon pudding with brandy sauce.

"It's a traditional menu. The same thing the Pilgrims had," deputy press secretary Larry Speakes said with a grin when questioned by reporters. Ann Allman, the Reagan's longtime cook, prepared the meal at the ranchhouse.

The guests were Maureen Reagan and her husband, Dennis Revell, Patti Davis and an unidentified boyfriend and the president's older brother and his wife.

Reagan interrupted his Thanksgiving vacation Wednesday for two telephone calls. He called Ambassador Paul Nitze on the eve of Nitze's departure to lead the U.S. delegation at the Geneva talks with the Soviet Union on theater nuclear weapons. The talks are scheduled to begin Monday.

Nitze will formally present the proposal Reagan made in a speech last week for the Soviet Union to dismantle its intermediate-range nuclear missiles in exchange for which the United States would not deploy 572 cruise and Pershing II missiles in Europe.

Reagan and Nitze underlined the U.S. intention to bargain in good faith and the seriousness with which the administration takes its proposal. The Soviet Union quickly rejected the proposal. Reagan received a telephone call from Chancellor Helmut Schmidt of West Germany who gave the president his assessment of Schmidt's just-concluded meeting with Soviet President Leonid Brezhnev.

All of the information in this story was given to reporters by Reagan press aides in Santa Barbara, 20 miles from the Reagan ranch.

The president has not been seen except on television since reaching his ranch Monday night.

He gave an interview to Barbara Walters that was broadcast to- night on ABC, and he could be seen at half time of the Detroit Lions-Kansas City Chiefs football game today, delivering a Thanksgiving message.

In remarks taped some time ago in the White House library, he asked all Americans to take time on Thanskgiving to offer a prayer.