Your working hours decide which slots appear on your booking page and calendar, so they're worth getting right. Set them under Settings → Hours & closures.
Working hours
Set the days you work and the times you start and finish. You can set different hours for different days — a shorter Friday, a late Thursday. These hours drive availability everywhere: the calendar, and the slots patients see when they book online. Nobody can book you outside them.
Breaks
For a regular break like lunch, set your working hours around it — e.g. finish the morning at 13:00 and start the afternoon at 14:00 — so no one can book over it. For a one-off gap on a given day, simply don't leave that slot open.
Closures
Add a closure for any date you're not working — bank holidays, annual leave, a training day. Give it a date and a reason. On a closed day your slots disappear from the booking page automatically, so patients can't book and you don't have to remember to block it manually. Remove a closure and the day opens back up.
Per practitioner
In a multi-practitioner clinic, each practitioner has their own hours, so a part-timer's availability is theirs alone — the booking page only ever offers times that practitioner actually works.
Changes take effect immediately, so if you add a closure the day drops off the booking page straight away.