Assistant booking rules and holidays rules
On-line available days rule
You need to consider that every time you set a rule for single assistant the booking process slows down as it require to process multiple calculations.
So use it only when strictly required.
Specific dates rule
you can add a rule where you can specify which dates this assistant is available for booking.
You can set "Holidays rules" for of your assistant but consider to limit them to a very short number as they would slow down the whole booking process.