This is the kind of small, exact post that saves someone a Saturday morning. The trap is exactly the one you name: 0 9 * * * looks like a weekday job until it fires on a Saturday and sends a reminder nobody wanted.

I like the generator sentence being literal, not clever — "at minute 0, at hour 9, on weekday 1-5" leaves nothing to guess. The timezone note is the other one worth checking before you trust any schedule: 09:00 in the daemon's timezone is not always 09:00 where your customers are.