In the interest of clarification from an Italian whose family actually does use the term "gravy," it refers to any tomato-based sauce, not one specifically served on Sunday or containing large amounts of meat. "Sunday gravy" is a term like "Sunday roast," i.e., lots of people may have it that day, but lots of people have it whenever.

But it's delicious no matter when it's served. Mmmm, (tomato) gravy....