At the top of the calendar, on the specific day, click the arrow pointing down
[
] next to your name. Now click
[Set Day as Full] to block it. To unblock it, uncheck the option again.
The blocked day is shaded in the diary.
Schedule an appointment on the first day that needs to be blocked. Give the appointment a title, for example "Vacation", and enter the start and end time. For example, 8:00 AM - 10:00 PM.
The selected days are now blocked for (online) appointments.