The message that stops most no-shows is the second one
A reminder a day ahead prevents the forgotten appointment. A reminder two hours ahead recovers the one they were going to skip.
The −24 h reminder does one job well: it catches the person who genuinely forgot. That is a real category, and it is why everybody sends one. But the client who is going to no-show usually knows by mid-morning, and a message the previous evening arrives before that decision is made.
The −2 h message lands after it. If it makes cancelling easy — one reply, no phone call, no apology to compose — a meaningful share of would-be no-shows convert into cancellations. A cancellation two hours out is not free, but it is a slot your waitlist can still take. A no-show is not.
You are not trying to stop them cancelling. You are trying to find out two hours earlier.
Which is the uncomfortable part: making it easier to cancel does increase your cancellation rate. It also increases your filled-chair rate, because the waitlist gets the slot instead of the floor. Watch the second number, not the first.