"middag" literally means "mid day", yes. That's also how it was used originally. The meal at the mid of the day was called "middag". Nowadays it's used for "dinner". It has something to do with it being the main meal of the day that shifted from lunch time to dinner time.
But in words as "förmiddag" (morning, lit. before mid day) and "eftermiddag" (afternoon, lit. after mid day) it still means "mid day" or "noon".
Yes I'm from the midwest and I've heard dinner and supper interchangeably, though I've never actually used the word supper. (If I really think about it I usually say "what's to eat?" or "what's for food?" or something shortened like that. I realize it's horrible grammar haha)
right, my mother is from Colorado and my Dad from Indiana (where I'm originally from though now I'm a Californian) but also lived in Texas for a while when younger. Maybe that's why we don't use one specific word?
It's really interesting. I come from France and we use "dîner" for the evening meal, "souper" was only used in the past times when they used to only eat a soup before going to bed, but they can still be interchangeable. Now I live in Switzerland and they use "dîner" for lunch and "souper" for the evening meal, and they are not interchangeable ^^' Confusing! I also thought that middag would be lunch. But the explanations here are very useful :)