According to her 1987 memoir And a Voice to Sing With, this is the first song Baez ever wrote. It's also one her most popular.
The song tells the story of Baez's sister Mimi Fariña and Fariña's relationship with music producer and '60s counterculture icon Milan Melvin. Early in their relationship, Melvin entered Fariña's bedroom through the window at night, as the lyrics describe:
Sweet Sir Galahad Came in through the window In the night when The moon was in the yard
The two were married in 1968 at the Big Sur Folk Festival but divorced in 1971. Both Fariña and Melvin died in 2001.
Sir Galahad was a knight of King Arthur's Round Table. He's characterized by his exceptional chivalry and moral bearing.
Baez first played the song publicly on an episode of The Smothers Brothers Comedy Hour on March 30, 1969. She later performed the song at Woodstock, where she was the final performer on the first day, going on around 1 a.m.