org-read-date-popup-calendar

org-read-date-popup-calendar is a variable defined in `org.el'.
Its value is

t

Documentation:
Non-nil means pop up a calendar when prompting for a date.
In the calendar, the date can be selected with mouse-1. However, the
minibuffer will also be active, and you can simply enter the date as well.
When nil, only the minibuffer will be available.

You can customize this variable.