How to activate calendar widget for Appointments?

To enable calendar widget for appointment booking, go to Settings -> Widget -> Product Page widget, and turn on radio button under Activate Widget as shown below.

After, Activating it the date & time selector will be shown automatically in the store.

Other articles to refer to for widget setup:
How to add Calendar widget to your store?

How to add Calendar Widget to other existing pages in your stores?

How to move Event Calendar Widget within the page?

How to create separate page for Calendar Widget in your store?