Variable: org-popup-calendar-for-date-prompt
org-popup-calendar-for-date-prompt is an alias and customizable for
org-read-date-popup-calendar, defined in org.el.gz.
Value
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.