Anneli Rufus unravels it by pointing to the past and various contemporary studies:
For most of history, meals were very variable. A medieval northern European peasant "would start his morning with ale or bread or both, then bring some sort of food out into the fields and have a large meal sometime in the afternoon," [Yale professor Paul] Freedman says. "He might have what he called 'dinner' at 2 in the afternoon or 6 in the evening, or later" — depending on his work, the season and other factors. "He wouldn't have a large evening meal. He would just grab something small and quick. Dinner back then tended not to be as distinct as it has become in the last two centuries."
And it tended to be eaten in daylight — not because eating earlier was considered healthier, but because cooking, consuming and cleaning up is difficult in the dark or by firelight. "People who were not rich tried to get all their meals eaten before dark. After electricity was discovered, initially only the rich could afford it," Freedman says. "From that point onward, one mark of being rich became how late you ate. Eating way after dark because you could afford electric lights was a mark of high status, urbanity and class."
The Dish 