References either Charles Manson, who led a series of murders that referenced the Beatles' "White Album," or to the group itself, whose song "Helter Skelter" appears on the album. (A "helter skelter" is also British slang for a child's slide.)
The birds flew off with a fallout shelter
Eight miles high and falling fast
One of the song's most obvious references, in this case, to the Byrds and their hit single "Eight Miles High," which some say is about drug experimentation, "high" being a euphemism for the euphoria induced by certain substances. (The "fallout shelter" reference is more than likely a direct attempt to comment on the threat of nuclear war and how it shaped a generation, although some insist it's slang for a drug rehabilitation clinic. Then again, "falling fast" can refer to a drug comedown or a bomb falling, so who knows.)
It landed foul on the grass
Some think this is in reference to an incident where the Byrds were arrested for possession of marijuana ("grass," as it was commonly termed then). In baseball, a ball that "lands foul" doesn't count.
The players tried for a forward pass
With the Jester on the sidelines in a cast
The reference to the Jester in a cast is almost certainly a nod to Bob Dylan's motorcycle crash of July 29th, 1966, which kept him out of the music scene for two years; intriguingly, it's often claimed that an exhausted Dylan exaggerated the extent of his injuries in order to get his fame back to a manageable level.
The "players," often considered the musicians of the time, are seen as trying to advance music forward in a American football metaphor. Some think they're the politically-minded youth of the day, hijacking rock and roll for their own purposes.
Now the halftime air was sweet perfume
Some have theorized this is a reference to either burning marijuana (being smoked, that is) or an ironic reference to tear gas, which was becoming a common sight to rioters and even peaceful protestors.
While the Sergeants played a marching tune
The reference to "Sergeants" and "Marching" almost certainly indicates the Beatles, and by extension the Summer Of Love -- 1967, when the album was released. This also fits in with the Dylan accident timeline.
We all got up to dance
Oh but we never got the chance
'Cause the players tried to take the field
The marching band refused to yield
Do you recall what was revealed
The day the music died?
Supposedly, this is a critique of the "Sergeants" and their insistance on taking rock in new, non-danceable directions. As to what was revealed, your guess is probably as good as Don's.
However, many insist that the "Sergeants" and "marching band" references all stand for the military and/or the National Guard, which would mean that McLean is referencing such incidents as the 1968 Democratic National Convention or Kent State.
We started singing...