Timing is everything

Our professor invited us to a party at his place on Saturday night at 9p.

I couldn't figure out why it was starting so late. (Not late-late, but late to start a party for someone older...)

And then I figured out, because then it's almost after Havdalah. Those who aren't shomer shabbes can come at 9ish (when it's still definitely light out), and those who are can come a little after and the party will still be happening.