Saint Monica and her son, Saint Augustine, found out that Saturday was observed as a fast day in Rome, where they planned to visit. It was not a fast day in Milan, where they lived. They consulted Saint Ambrose who said "When I am here (in Milan) I do not fast on Saturday, when in Rome I do fast on Saturday. '' That reply is said to have brought about the saying "When in Rome, do as the Romans do. ''
where does saying when in rome come from
Saint Ambrose