You can have as many different schedules in FlexBooker as desired, and each employee can even have multiple schedules.
Each schedule can be set with both a start, and optionally an end date.
If there are any days that two schedules for the same employee intersects, then don't worry, the system won't allow double-bookings, it will just allow booking for any hours that there is availability in either schedule.
So, let's say we want to open up two extra days during the week of Christmas only:
1) Click on the "+" sign in the Flexible Schedules header.
2) Choose the employee on the first step, then on the second step, make sure you choose the date range for the week you want to update:
3) Drag and drop the hours you want to add into the calendar.
Now finish the schedule creation process and those days or hours will now be opened up!