The BBC has a feature on the history of Unix, in celebration of its 40th anniversary this month. In 1969, Ken Thompson and Dennis Ritchie had been meaning to write a new operating system for a while, but it wasn't until August that Thompson got some time to himself. Then he hammered out the core of Unix in one month:
[In] August 1969, Ken Thompson's wife took their new baby to see relatives on the West Coast. She was due to be gone for a month and Thompson decided to use his time constructively — by writing the core of what became Unix.
He allocated one week each to the four core components of operating system, shell, editor and assembler.