So to me, "supper" and "dinner" were always interchangable.
But in the dictionary, "supper" is defined as "a light evening meal; served in early evening if dinner is at midday or served late in the evening at bedtime."
"Dinner," on the other hand, is defined as "the main meal of the day served in the evening or at midday."
So in other words, "dinner" can be "lunch" and "supper" can be "dinner."
Or, as Homer Simpson once said, when asked how he managed to gain so much weght, "I discovered a meal between breakfast and brunch!"