Dinner's the main meal of the day, whenever it takes place. Unless it's at teatime, in which case it's high tea.

So in the week it'd be dinner at lunchtime, and at weekend maybe it'd be dinner a bit later than teatime, on a Sunday anyway. And Christnmas Dinner is of course at lunchtime - except that it always falls behind schedule and ends up halfway to teatime. Lunch is what you pack in a lunchbox, if you aren't going to get a proper meal.

Confusion can arise when families with different understandings about such things - like when we had someone accept an invitatiion to Christmas Dinner under the impression it was in the evening.