Locking will make a day not available for a booking/appointment. To perform this operation, you need to click close under the title which contains the date of the day do you wish to lock.
Note that if the day that you close has already an appointment/booking, it will remain active.
The case when the date which was previously closed for booking by using the close the day function, needs to be open for booking again. To perform this operation you need to click open under the title.