If you have an event that is happening for an entire day, you can create an event as an All-day event on Zoho Calendar and update your free/busy status.
Enter the Event title, Date and Time of the event.
Check the All day check box below the Date and Time field.
Enter other details of the event.
Click Save.
An All-day event will be denoted by a solid color band on the date grid of the event. The color of the solid band will be based on the Calendar color code or the chosen event color.