How to run a procedure on entering a specific cell?

I am trying to pop up a calendar control to enter a date into a
specific cell. For example: when the user moves the cursor to "B5", the
calendar control should become visible. The user then clicks on a date,
which is updated in cell "B5"

My problem is I do not know how to detect when the user selects a
specific cell in order to run the procedure. Is there a way?