Lunch is the noon meal. Supper is the evening meal. Dinner is the main meal, regardless of when it's eaten. So, most weekdays, dinner and supper are interchangeable... but Sunday Dinner might be lunch. (I'm from the Midwest.)
ETA: my mother grew up in a farming community and said that lunch was often the main meal for them, and they called it dinner. Supper was a lighter meal in the evening.