In season 1 episode 7: Red in the Face, there was this exchange:
Roger: What are you up to tonight?
Don: I'm on the 5:31.
Roger: Reservations at home. I've had those.
What does the 5:31 refer to? I tried to search its meaning and the closest thing I could find was maybe a reference to Matthew 5:31. I assume it just means what Rogers replied: "Reservations at home", but where did this saying come from and was it used frequently back in the 1960s?