The customer calendar is accessible through the main menu at Calendar > Customer
It is a Gantt style view of bookings by customer. Each booking is represented by the customer name, booking status (colour), and will span the number of days for the booking.
If your item is configured with a nightly allocation, the booking will overlap slightly into the day following the last day of the booking (see image below). This represents the check-out day which is not actually part of the paid booking. That day is still available to be booked again by somebody else.