The word 'weekday' means Mon, Tues, Wed, Thurs and Fri. That's clear.
According the dictionary.com, 'weeknight' means 'any night of the week, usually except Saturday and Sunday'. According to Miriam- Webster, it's 'a weekday night', with weekday defined as above. So it's Mon-Fri nights.
But I've just realised I always use it to mean Mon-Thurs nights, or Sun-Thurs nights, not Mon-Fri nights. As in:
"I don't go out on weeknights, because I have to work the next day."
Anyone else using it like that? And if not, can you think of some other expression for Sun-Thurs nights?
There's also "school nights", which does mean Sun-Thurs nights, but I've never liked using that except for schoolchildren.