When you were growing up, your parents or grandparents may have referred to lunch as "dinner" and dinner as "supper." This can be confusing for younger generations who use "dinner" as a blanket term for all evening meals, but the word doesn't actually refer to a specific time. It just means that it's the main (or biggest) meal of the day.
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Up until the start of the 20th century, the main meal was what we now refer to as "lunch," which was formerly called "dinner" because that was when Americans ate the largest meal featuring multiple courses, grand portion sizes or both. The evening meal was called "supper," which was much lighter and more informal. Instead of cooking more food, people traditionally just ate soup or leftovers from dinner.
The meanings for both words have shifted over time; "dinner" ultimately comes from a Latin word with the same literal meaning, oddly, as "breakfast" - it was, after all, the first substantial meal of the day - while "supper" stems from a Germanic word that literally meant something like "sip soup" and is related to both of those words as well.
But midday dinner and evening supper were the most prevalent terms in agricultural communities during the 18th and 19th centuries. Farmers would fill up on dinner around noon to carry them through work, but that all started to change when people began working away from the farm and weren't able to return home to eat in the middle of the day, food historian Helen Zoe Veit told NPR. "Supper" became "dinner" because evenings became the only time working class families were able to gather for a meal.
In this day and age when someone invites you over to eat, you can safely assume that "lunch" means midday and "dinner" means to arrive in the evening. The word "supper" has faded over the years, but it could be wise to clarify with older generations because being timely is just one of many old-fashioned etiquette lessons your grandma wishes you knew.