where did the idea of a sunday dinner come from?!

Sunday Roast dates back to when the squire would treat his serfs to a meal of roast oxen every Sunday to reward them for the week's work.[1]

Technically, the term Sunday Dinner does not mean necessarily that an actual roast has to take place. Such things as sausages, bacon and barbecued meat can be incorporated into a Sunday dinner. The only rule being that the dinner is held on Sunday.

people who worked in "the big houses" got the leftovers from the masters saturday dinner party as a treat
after doing their sunday morning chores, the cook would re heat and lay on a dinner for the workers
who got a sunday afternoon off

Typically people would work all week and the only time they could get together for a hot meal was on Sunday and usually after church.

I'd say it came from Henry VIII - the meat and veg bit. But I don't know where the Sunday roast bit came from, maybe it was just taken up?