As Marny (also from Hannover, Germany !!!) has already posted it's Frühstück, Mittagessen and Abendbrot here. Abendbrot if it's cold, Abendessen if it's cooked.
Supper in the evening, regardless of how formal the setting... Dinner was a formal mid-day meal, regardless of the day of the week, otherwise it was lunch...
I grew up in Toronto, where we had lunch and dinner. Now I live in New Brunswick, where most people have dinner and supper. Lunch is what you eat before you go to bed.