Every once in a while, I will hear someone of a couple of generations ago refer to the noon-time meal as Dinner and the evening meal as Supper. I certainly understand what they mean, but it’s a little bit of a context switch. I was always under the impression it was just different back then, but it turns out, reading this history of mealtimes, that the concept of what “lunch” and “dinner” were pretty fluid, even 50 years ago.
In general, dinner used to be the main meal of the day. Due to the fact that there wasn’t much in the way of artificial lighting for the majority of the population until the late part of the 19th century, the main, largest, most elaborate meal had to be when there was the most daylight, which would be around the time we typically have lunch today.
Lunch was created by upper-class ladies as a time to eat between breakfast and what was becoming a later and later dinner time, more around the time we typically have it these days. It used to be a ladies-only affair, though that also changed as the world became more industrialized and people started working through the afternoon.
The meals used to be named around how big they were or how elaborate they were. Now they are named strictly by time: breakfast is the morning meal, lunch is the afternoon meal, and dinner is the evening meal. Read the whole article for a lot more detail.