supper or dinner, what do you call that meal?

[Shepherd, Jerry K.]

Hello, I have a small problem.  A couple weeks I went to visit my
grandmother.  She asked that I come by for dinner.  So, on that day, just
before 6 PM I stopped by.  However, she had already eaten. In fact, she ate
dinner for lunch.  How can this be?  She gave me quite a verbal lashing,
stating that dinner is eaten at night while lunch is eaten at noon.  She
told me to stop "smart mouthing" and come in and eat leftovers.

As the expert you would know this distinction,  what's the difference
between supper, dinner and lunch?

thanks for any help you can provide
Jerry