When are reminders sent and can I change the text?

There are two reminders and their timing is fixed: one 24 hours before the appointment and one 2 hours before. They are calculated in your business time zone, not the recipient one.

A reminder only goes out if the client already knew the time before its window opened. Someone who books today for 5 hours from now does not get the 24-hour one, because they have just received the confirmation with that time, but they do get the 2-hour one; someone who books for an hour from now gets neither. If you move an appointment, the reminders count again for the new time with the same rule: the "Appointment moved" notice has already given them the time. Appointments you import from your previous software do get both, because the client already knew their time.

What you do decide, in Settings › Communication › Automatic notices, is which notices go out and through which channel: appointment created, confirmed, cancelled, moved and those two reminders, each with its channels on or off.

Right there, under "Reminders & consent", you can enable a no-notification window and choose what happens to the reminders that fall inside it: "Postpone" sends them as soon as the window ends and "Don't send" skips them. Nobody wants their appointment reminder at three in the morning.

The wording of the notices is written by BookflowR and already translated into the client language: it cannot be edited today. If you need to say something of your own in every appointment (where to park, what to bring), the practical place is the service description, which does travel with the booking.