Calling off a day because you are ill meant opening every appointment separately and cancelling it separately.
What is new
In the appointment list you can tick rows. A bar then appears at the bottom with what you can do with that selection: cancel, confirm, export, or move on to Rebook.
When you cancel, the confirmation dialog asks whether you want to inform customers, and that then applies to the whole selection.
Nine worked, two did not
That is the normal outcome, not a fault. Move ten at once and two of them run into something else in the calendar, and you hear exactly which two and why, and those two stay ticked so you can pick them up afterwards. The eight that did work are simply done.
Recurring appointments cannot have their status changed in bulk. On an appointment like that you first have to choose whether you mean just this one, this one and the ones after it, or the whole series, and that is a question a bulk confirmation cannot ask.