She was waiting for a dinner date with Cat Stevens, who was 40 minutes late. Exasperated by his tardiness, Simon didn't waste time fuming. Instead she took her anger out by writing one of the biggest hits in just a few minutes.

"Anticipation" was about that very thing - anticipating a date who never seemed likely to show up at all. 

Waiting For Cat Stevens Inspired Carly Simon To Pen This 1971 Hit In Under An Hour