I have always used the term ‘dinner’ to mean the evening time meal, so I was surprised to learn that many Americans use it to mean ‘lunch’. In parts of the country where their dinner is my lunch the term supper is used to mean the evening meal. This discrepancy comes from the idea that dinner is the biggest meal of the day. In some places, especially rural ones, the largest meal of the day comes closer to noon. This midday feast is meant to give hard workers sustenance to last until dusk. Supper comes from the word ‘soup’. It was common to put the scraps in a pot and boil them all day until there was soup at the end of the day.
Dinner has an interesting etymology as well. When the term dinner was invented there were only two meals of the day. Here is what wikipedia says about the topic: The word is from the Old French (ca 1300) disner, meaning “breakfast”, from the stem of Gallo-Romancedesjunare (“to break one’s fast”), from Latin dis- (“undo”) + Late Latin ieiunare (“to fast”), from Latin ieiunus (“fasting, hungry”).
Today we eat breakfast, which is following the etymology of dinner. The moral of the story is that what we eat and when we eat varies between region and era.