President Carter will return from Camp David Sunday and deliver a nationally televised speech from the Oval Office at 10 p.m., the White House announced yesterday. The speech will deal with issues that Carter "feels are important to the nation, including energy," the announcement said.

Last night Carter added another chapter to the sometimes bizarre story of his "domestic summit conference" when he flew by helicopter to Pittsburgh to discuss with private citizens the domestic policy choices facing him.

Carter's Sunday speech is the first step in a carefully coordinated media blitz that will mark the president's return from 12-day stay at his Maryland retreat for a crucial series of public appearances.

On Monday, the president is to fly to Kansas City, where, in a speech to the National Association of Counties, he will provide "additional specifics on energy," the White House announcement said. Later that day, he is to go to Detroit, where he is to speak and answer questions before the annual convention of the Communications Workers of America.

This flurry of activity is to continue into next week, with Carter scheduled to meet with congressional leaders, staff and other executive branch officials.

The president, accompanied by Rosalynn Carter, left Camp David at 6 last night, and returned about 10:30.

Deputy press secretary Rex Granum said Carter spent about 90 minutes at the Pittsburgh home of William and Betty Fisher talking with about a dozen people. Granum quoted Carter as saying it was "an interesting and informative conversation."

Granum said the Pittsburgh residents did not know they would be talking to the president until "shortly before his arrival."

The announcement solved a 3 1/2 hour mystery about Carter's whereabouts. Granum earlier had said he could not reveal Carter's location because of efforts to keep the meeting private.

Granum said Carter decided a week ago to include in his domestic summit a visit to a group of citizens somewhere within helicopter range of Camp David.

The president spent part of yesterday working on his planned speeches and conferring with White House advisers on the energy policy he is to unveil in detail in Kansas City.

The importance White House aides attach to the public reaction to what Carter has to say next week was illustrated by an invitation to several influential columnists and television commentators, including the anchormen of the three television networks, to go to Camp David today for a special "background briefing" on the president's intentions.

Carter will emerge from the tranquility of the Catoctin Mountains Sunday at what is probably the most critical point of his presidency.He is at a low point in the public opinion polls, and faces growing skepticism in his own party that he can salvage s second term in the White House.

The political stakes for the president in what he has to say Sunday and Monday and how it is received have been increased all the more by the events of the last 10 days. His decision to cancel a scheduled energy speech last week and to convene the extraordinary "domestic summit conference" at Camp David was perhaps his biggest political gamble as president.

The drama and mystery surrounding the summit have focused public attention on the nation's domestic problems, but they have also raised public expectations of the outcome at a time when polls suggest that many Americans have all but given up on Carter's ability to manage the government and lead the country.

Twice before the president has taken similar gambles - when he invited the leaders of Egypt and Israel to Camp David for peace negotiations, and when he flew off to the Middle East to nail down the details of an Egyptian-Israel peace treaty. Carter won both those gambles despite considerable skepticism at the time.

However, those earlier efforts involved negotiations with other heads of state, and not the president's ability to inspire national confidence in his domestic policies and leadership abilities.

White House offials who have been left behind in Washington and kept largely in the dark on the events at Camp David concede that Carter will run some risks when he reemerges in public Sunday night.

Speaking of the "domestic summit," during which more than 100 people with diverse backgrounds have been flown to the presidential retreat to confer with Carter, one official said:

"I don't think it was calculated from the beginning, but people up there may have realized during the process that it would dramatize what comes out of it. But that's risky business, because if you don't come out with something, it dramatizes that, too."

White House press secretary Jody Powell has said repeatedly that Carter canceled the energy speech last week because he wanted to deal with the energy question in a "broader context."

The president apparently will attempt to do that Sunday night in a speech that may tie the nation's energy problems to what Carter has described to some of his Camp David visitors as a national malaise he sees afflicting the American people.

White House aides said Carter will discuss energy Sunday night and may mention the highlights of his new energy proposal. However, the details of those proposals will be saved for the speech in Kansas City Monday, guaranteeing a second day of saturation news coverage.

Both the Kansas City and Detroit appearances have been scheduled for several weeks, and provide convenient vehicles for the president to demonstrate his concern over domestic issues at the conclusion of his stay at Camp David.

It remained unclear yesterday whether in any of his public appearances Carter will have anything to say about the fate of Energy Secretary James R. Schlesinger Jr., rumored to be on the verge of resignation or out-right ouster.

White House deputy press secretary Rex Granum yesterday denied one published report that Schlesinger's job has been offered to John D. deButts, the retired board chairman of American Telephone and Telegraph Co. But beyond that, the White House had nothing to say about Schlesinger's future.

Schlesinger has told reporters that he does not want to remain in the administration beyond the fall because he considers himself a liability to Carter's reelection chances. But the swirl of rumors about his future, which Schlesinger aides attribute to White House officials anxious to see Schlesinger fired, could move up the date of his resignation. CAPTION: Picture, Heading for a Camp David-bound helicopter are Patricia Harris, secretary of housing and urban development, Detroit Mayor Coleman Young, Idaho Gov. John Evans, second from right, and Atlanta Mayor Maynard Jackson. UPI