Perhaps you were going to close your business for a day for maintenance but the plans have changed so you want to be able to start accepting bookings for the previously-closed date. In order to remove the date navigate to the closed dates area at Settings > Closed Dates. You will be presented with a list of closed dates. 

Select the date you wish to remove and select Remove. You have the ability to select multiple dates at once to remove.

Once the date(s) to re-open have been removed from the list select Save Changes for the changes to take effect.