"I'll be glad when this day is over," Nancy Reagan told her chief of staff, James Rosebush, yesterday morning. "This is no picnic."

In the end, it was a long and exhausting day for the first lady, who was applauded for her strength and grace by many close to her, including Vice President Bush, for the way she handled the news that the tumor removed from President Reagan's colon was malignant.

Just hours after the first lady received the medical report at Bethesda Naval Hospital, she was back at the White House to receive the diplomatic corps and sit for the 100th-anniversary concert of the Boston Pops on the South Lawn.

She said very little about the president in her prepared remarks, except to note that he wished he could be there and sent his regrets. Afterward, she invited her 400 guests into the White House and stood for an hour shaking hands.

Swedish Ambassador Wilhelm Wachtmeister, speaking for the diplomats, told the first lady and the crowd gathered in the sweltering heat, "We are particularly grateful to you, Mrs. Reagan, for being with us here today though your thoughts must be elsewhere, with your husband, and so are ours.

"We are delighted the operation has been successful. We look forward to soon having the president among us again."

During the White House reception following the concert, Bush privately commended Mrs. Reagan as being "amazing" in her ability to deal with the pressures of the past few days.

"Considering what she has been through, it has got to be hard," he said.

Asked if he planned to visit the president in the hospital, Bush said. "No. I think he needs rest." He then added, "Sometime, but not now."

Health and Human Services Secretary Margaret Heckler said of Mrs. Reagan: "This is her finest hour. She has shown her mettle . . . The love those two have has sustained us all."

"Her spirits seem remarkably good," said Sondra Gotlieb, wife of the Canadian ambassador and a guest at the reception. "When I first saw her tonight, I hadn't heard the news, and I looked at her face and said, 'The news must be good.' "

Mrs. Reagan left the reception after the receiving line ended.

Said Rosebush later, "If you asked me to describe her feelings, her mood right now, I'd say she is drained."

Rosebush said he thought Mrs. Reagan was trying hard to keep her feelings under control because "she wants to be strong in front of him Reagan ."

He said the first lady planned to spend the evening "taking a bath, having dinner and calling family."

The first lady arrived at the naval hospital about 10:45 a.m. to wait with the president for the results of his laboratory tests. She carried a silver balloon bearing the greeting "GET WELL."

She had lunch with the president and finally, at 2:40 p.m., doctors took her and White House chief of staff Donald Regan into a sitting room adjoining Reagan's room to tell them of the findings.

White House spokesman Larry Speakes said yesterday that she looked straight ahead "without blinking" as doctors gave the results of the tests, and "asked clear, concise questions of them and listened very intently as they spoke."

"She took it very calmly," said Speakes, who also was with the first lady when she spoke to the doctors. "She said she was relieved to hear the report that all the cancer was removed." Doctors have said that the chances are "greater than 50 percent" that the president will have no recurrence of cancer.

Speakes said doctors spent more than 30 minutes with Mrs. Reagan, going "into considerable detail."

She was also described by one of Reagan's doctors as "quite gratified" that the cancer had not spread elsewhere.

Aides said Mrs. Reagan did not cry. Doctors told her in advance of the president because they felt she could be more supportive that way.

When the president heard the report a few minutes later, Mrs. Reagan was with him. That session lasted about five minutes and afterward, Speakes said, the president's doctors spent another "five to 10 minutes" with the first lady before she returned to the White House.

Mrs. Reagan's public effort, at least, seemed to be a business-as-usual approach. Though Regan reportedly tried to persuade her to let Bush stand in for her at last night's reception, she was adamant about being there to represent the president. She also was determined to go on with her visit to the USS America off the coast of Norfolk tomorrow, although she has telescoped her tour of the aircraft carrier and her shipboard briefing on the Navy's program against drug abuse into a single day rather than an overnight visit.

What seemed to be missing in her life over the past three days were not only the Reagan children but also Reagan friends, many of whom, like former aides Michael Deaver and Nancy Reynolds, were traveling abroad on business. Rosebush said there had been calls of concern and support from all of the Reagans' longtime California friends and that Mary Jane Wick, wife of United States Information Agency Director Charles Z. Wick, volunteered to fly back from Los Angeles to be with Mrs. Reagan.

"Essentially Mrs. Reagan's alone," Rosebush said. "I suppose anybody would rather have their friends comforting them, but in the end it's something you have to go through yourself."

Her stepbrother, Dr. Richard Davis, who sat with the first lady during the surgery, is expected to return to Washington today from his home in Philadelphia and spend the next two nights at the White House, Rosebush said.

Michael Reagan, Reagan's son from his first marriage to actress Jane Wyman, said yesterday that he had spoken to Nancy Reagan before his father underwent surgery and had offered to fly to Washington to be with her.

"On the other side of it," Michael Reagan continued by telephone from Los Angeles, "if anybody from the family had gotten on the plane to fly back there it would have looked odd. Any other family can go visit their dad and that's fine, but when the first family does, everybody says, 'Okay, what aren't they telling us?' "

The Reagans' son Ron, in a television interview yesterday morning before test results were announced, called his mother "a worrier, you know, so naturally she's probably taking this harder than he is." Of his father, he said: "He's got the constitution of a horse, so I don't think anything's going to keep him down."

Rosebush said in the morning he thought that in some ways it is "harder on the spouse" than on the patient, and that in Mrs. Reagan's case she is trying to be "up, be cheery, but is having to face reality." He said the first lady's current ordeal reminded him of when her father, Chicago neurosurgeon Loyal Davis, was dying and she spent nearly two weeks in Phoenix with him.

Rosebush said that at that time, "she slept right there in the hospital. Here she is again, in a hospital."