By default, the online booking hours are the same as the store opening hours. Enabling online booking hours allows the booking times offered on the online booking page to differ from the core store opening hours.
Enable online booking hours by going to Settings -> Admin -> Features and Customization and turning on the online booking hours feature.
The lozenge will then appear in the Online section in settings as shown in the screenshot below.
Then set the hours you wish to allow customers to book online.
Features and Customization page