In what world??? I've been all over the world for the last 73 years and this is the absolute first time I have ever heard spaghetti sauce referred to as Sunday gravy. Hell, for at least 30 years, in the middle of all that, Wednesday's were considered Prince Spaghetti Day. What would that have to do with Sunday? I don't know, maybe I didn't get the memo. I've even lived down here in the south where they have a penchant for naming thing like they were all insane, yet no mention of Sunday Gravy.
I even worked with a guy back in the 60's that was 140% Italian and he gave me a recipe for Spaghetti Sauce (note not Sunday Gravy) that had everything including the kitchen sink in it and it took a full 12 hours of simmering to make. Required imported from Italy whole tomatoes. tomato paste, pork chops, homemade Italian sausage and to many herbs and spices to even remember. When my wife left she took the recipe with her and probably decided that slip of paper was taking up to much room in her 3X5 card file. We can't find it in stuff that she left behind so we have to assume she tossed it away. That recipe makes the rest of you look like people opening up a can of spaghetti-O's. Sunday Gravy, indeed! Was that something that Gen X came up with.
In the south Sunday Gravy could easily be Sausage Gravy which is a staple here along with biscuits. I wonder if someone heard the name Sausage Gravy and thought they said Sunday Gravy. Still doesn't fit in but at least it provides an otherwise unexplainable reason.