Although the song was recorded in 1971, it was written around 1969, making it ten years from the date of the crash.
And moss grows fat on a rollin' stone
Almost certainly a reference to priviliged and pampered rock stars like The Rolling Stones, who'd "grown fat" on rock and roll. Also refers to an old adage about a rolling stone gathering no moss... in the mind of the singer, rock had picked up lots of weight along the way, and none of it good. Buddy Holly himself references the "rolling stone gathers no moss" cliche in his own song "Early In The Morning."
Other theories hold that the rolling stone is original Stones member Brian Jones, who was long dead when the song appeared, or Bob Dylan (famous for his hit "Like A Rolling Stone") or Holly himself.
But that's not how it used to be
When the Jester sang for the King and Queen
Probably the most debated lyrics in the entire song. The King is almost certainly thought to be Elvis, since his nickname (The King of Rock and Roll) was well-established by 1971, but there's no clear consensus on who the "Queen" is... possible contenders include Connie Francis, the biggest selling female star (but hardly a rocker), or Priscilla Presley, Elvis' wife (whom he didn't marry until 1967).
In a coat he borrowed from James Dean
And a voice that came from you and me
The Jester is widely considered to be Bob Dylan, in part because he wears a red windbreaker on the cover of his 1963 album "The Freewheelin' Bob Dylan" that is remarkably similar to James Dean's jacket in the 1955 film "Rebel Without A Cause" (both major cultural landmarks, each in their own time). This also leads some to believe that the King and Queen are none other than President John F. Kennedy and his wife, Jacqueline Kennedy; Dylan performed at a Martin Luther King rally in August 28, 1963, which the couple watched on television.
It's rumored that Dylan sang at a British oub called "The King And Queen," and also that he sang for the Queen of England herself.
Some have also noted that the windbreaker Dean wears in the film is given to another teen, who dies while wearing it. Dylan was also badly injured in a motorcycle crash while Dean died young from a car wreck.
Oh, and while the King was looking down
The Jester stole his thorny crown
"Thorny crown" is naturally a reference to Jesus Christ, which gives new meaning to the Kennedy theory, since the President was assassinated, some say as part of a wide conspiracy by corrupt men, in 1963. More popularly this couplet is thought to refer to rock's growing social consciousness, as folkies like Dylan became the voice of a generation while old rockers like Elvis became increasingly irrelevant. (This gives further meaning to a "voice that came from you and me," especially since Dylan was a major fan of populist folk legend Woody Guthrie.)
The courtroom was adjourned
No verdict was returned
A murky couplet that may have no real meaning... certainly there are any number of famous Sixties trials that can be attributed to this verse, but none that make any real sense in the context of the song. It's been theorized that this is a metaphor for the growing inability of America to form a consensus on anything, even the music that had once held them together.
And while Lenin read a book on Marx
A play on words, specifically John Lennon's utopian (and some would say communist) ideals, which would tie into the ones espoused by the founders of Socialism, Karl Marx and V.I. Lenin.
The quartet practiced in the park
The "quartet" is almost universally considered to be the Beatles, as will become clearer later. However, some consider the folk quartet the Weavers to be the subject, which would make sense, considering that McLean himself was a folkie who knew members of the group personally. Note also that the Beatles are called "the Sergeants" later, which would seem to preclude them also being referred to as the "quartet."
And we sang dirges in the dark
Dirges are funeral songs, often sung in the dark. Another metaphor for death and dying. Possibly the dirges were sung for the Kennedy brothers, or Martin Luther King.
It's also possible that the darkness in question is the infamous East Coast blackout of November 9th, 1965 -- McLean, remember, was a New Yorker.
The day the music died
We were singin'...