I think in that case we can agree on that dinner refers to the main meal of the day. Yes, every doctor will tell you that the most important meal of the day is breakfast, but usually dinner is the main meal where we eat something warm, sometimes with many courses and usually more or less elaborate (breakfast for me is mostly just a coffee with two toasts). Depending on your daily time schedule and lifestyle, you might eat dinner more or less early and instead eat something in between.
I wrote "We eat salmon at dinner tonight" and was marked wrong. It seems to convey the same meaning. Is it because "till" won't stretch that far? One other unrelated question I've been meaning to ask. I once learned kväll as evening and natt as night but often see them used interchangeably. So maybe natt is for after midnight but up until then either works?