WASHINGTON — From the hillside grave site in Arlington National Cemetery, Army Sgt. Keith Clark could see John F. Kennedy’s vast funeral cortege crossing Memorial Bridge toward him.

He could see the flag-wrapped coffin, the six white horses pulling the caisson, the endless line of black automobiles bearing the world’s dignitaries. He could hear the cadence of the muffled drums.

It was Nov. 25, 1963. Clark, 36, the Army bugler assigned to sound taps at the funeral, had been waiting in the cold for hours. A perfectionist and superb musician, he had just played taps for the president on Veterans Day two weeks earlier.

Now he had the most important and solitary task of his life: Sound the 24 notes of the venerable melody that would close the nation’s wrenching, four-day farewell to its assassinated president.

But the pressure, the cold and the wait told on Sgt. Clark that day 50 years ago this month.

And with the whole nation and much of the world listening, Clark fumbled the sixth note of taps, which falls on the word “sun” in the lyrics, “Day is done. Gone the sun …”

Some said it sounded almost like a sob, befitting the moment.

Back home, in Arlington, Va., though, his wife and four daughters, watching TV in the basement, let out a groan.

Clark went on to finish flawlessly. His flub has gone down in bugling history as the poignant “broken note” of the Kennedy funeral. It was a testament to the anguish of the day, and to the human truth that under duress, even the best can make a mistake.

After the funeral, Clark got letters from all over the country, sympathizing. One was from a 9-year-old Ohio boy named Eddie Hunter, who played in a school band.

“Anybody is bound to make a tiny mistake in front of millions upon millions of people,” he wrote.

Clark, who kept that letter, died in 2002 at the age of 74.

On Saturday, family and friends — including Hunter, now 60 — plan to join the U.S. Army Band and 100 buglers to pay tribute to Clark at Arlington Cemetery, where he is buried on a commanding hilltop.

It seems a fitting salute, one of his daughters said, to a dedicated musician who, had he nailed taps that day, might be utterly forgotten.

“The JFK funeral, the actual funeral ceremony … involved some of the most iconic moments of the entire four-day tragedy,” said James Swanson, whose new book, “End of Days,” chronicles the assassination.

“One of the most memorable sights and sounds at President Kennedy’s funeral was the broken note of the bugle,” said Swanson, who is scheduled to speak at the Clark commemoration.

“That was really the climax of that weekend,” he said. “Nonstop television for four days … And after all the words — millions of words by commentators, published in newspapers, published in magazines — the tragedy ends with a single bugle call.

“That broken note sort of symbolized what that weekend meant to the American people,” he said. “It’s like a human cry. It’s like the bugle was weeping. … It was really the perfect ending to those four days.”

But to Clark and his family, it was a mistake.

“My dad had played taps thousands of times, and I mean thousands of times … and never missed a note,” said his eldest daughter, Nancy McColley, 64, of Port Charlotte, Fla. He “always strove for perfection.”

She said she has a memory of him coming home and flinging his hat in frustration.

His wife, Marjorie Clark, 90, remembers the children confronting him, and one saying, “Why did you make a mistake?”

Clark, himself, later said: “I missed a note under pressure,” according to a 1988 Associated Press story. “It’s something you don’t like, but it’s something that can happen to a trumpet player.

“You never really get over it,” he said.

After the funeral, Clark recalled, Arlington buglers missed the jinxed note regularly.

In 1963, Clark was the principal bugler in the Army Band.

He was “THE guy, who is to do all the big ceremonies,” said Jari Villanueva, a retired Air Force bugler and bugle historian.

“They always pick the best player, the person who can stand up to the pressure of high-profile events,” he said.

Clark played Memorial Day ceremonies, Veterans Day ceremonies. He played at Arlington funerals and at the Tomb of the Unknowns.

There, on Nov. 11, 1963, he sounded taps a few feet from President Kennedy — 11 days before the Nov. 22 assassination in Dallas.

Clark, a native of Grand Rapids, Mich., “was a prodigy,” Villanueva said. His father was a professional flute player. And Clark attended the University of Michigan and the Interlochen Arts Academy, near Traverse City.

He joined the elite, Fort Myer, Va.-based Army Band, “Pershing’s Own,” in 1946. He met his wife, and they raised four daughters in Arlington County.

Villanueva, who said he interviewed Clark by mail and telephone before he died, said Clark was a devoutly religious man and Sunday school teacher who had a collection of 9,000 hymnals.

Indeed, Clark was in his attic library with his collection when his daughter, Sandra Masse, then 10, came home from school and called up the stairs that the president had been shot.

Learning of Kennedy’s death, and figuring he might be summoned to duty, Clark immediately went out and got a haircut.

But when the tragic weekend passed with no call, he thought funeral organizers might have gotten a Navy bugler because the president had served in the Navy, Villanueva said.

Then, at 2:30 a.m. Monday, the day of the funeral, the phone rang at Clark’s home with the orders: He would sound taps at the funeral.

Amid the frenzy of making arrangements, organizers didn’t realize until the last minute that they had no bugler.

But Clark, a balding man who wore dark horn-rimmed glasses, was ready.

When he reported to the cemetery in his dress blue uniform and white gloves, and with his elegant brass bugle, Villanueva said, he was shown an X on the grass where he was to stand.

He was alarmed because it was within a few feet of the rifle party, which would be firing practically right in his ears just before he played.

Then, he was told he was to play into a microphone. He refused. Villanueva said Clark always played for the widow at a funeral and didn’t like to use a microphone.

He began his wait.

“His spot is located on the slope below the Custis-Lee Mansion,” Villanueva said. “He’s got the perfect view of what’s happening. He can actually see the Memorial Bridge from there. So you can hear the procession coming. You see the procession coming.”

Clark watched the cortege enter the cemetery, Villanueva said. He watched the body bearers lug the mahogany coffin to the grave and heard the gravelly-voiced Roman Catholic Cardinal Richard Cushing pray over “our beloved Jack Kennedy.”

The deafening rifle volleys were fired, and then it was time for taps.

“For any bugler, when the time comes … everything stops,” Villanueva said. “Everything becomes very quiet. It is just you.”

Clark raised the sparkling bugle with gloved hands and began.

Villanueva said Clark often thought at such moments of the Bible verse from St. Paul’s first letter to the Corinthians:

“In a moment, in the twinkling of an eye … the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.”

When Clark finished playing, he whipped the bugle under his left arm and saluted.