It's very simple to allow bookings on a day or time that you would normally be closed.
From the staff calendar:
If you're trying to book from the staff/stylists calendar side of things you just go ahead and click on the time/day that you want to book and the system will allow this.
As a staff member you are able to book outside normal operating hours.
From the client side:
You can allow clients to use the online booking system to book appointments on days/times when you would normally be closed (around holidays, before your vacation, wedding parties etc).
To do this go the Settings tab on the left side menu.
Under Your Settings, click on Hours/Vacations. This will bring up your options on the right side of the screen.
On the right side of the screen you will want to click on "special hours", where you can change the hours for a single day.
A new window will appear where you can select "+add special hours".
This will open up a new window which will allow you to chose the date and hours that you would like to be open for online bookings.
Fill in your desired dates and times, hit the Save button, and you're all set.