This expression comes back from a letter of St. Ambrose in 387 A.D. addressed to St. Augustine.
St. Augustine had arrived in Milan to take up his role as professor of rhetoric for the Imperial Court. Saint Monica and her son, Saint Augustine, found out that Saturday was observed as a fast day in Rome, where they planned to visit. However it was not a fast day in Milan, where they lived.
Seeking guidance, St. Augustine wrote to St. Ambrose, then Bishop of Milan, who replied “When I go to Rome, I fast on Saturday, but here [in Milan] I do not. Do you also follow the custom of the Church where you are, if you do not want to give or receive scandal.”
That reply is said to have brought about the saying “When in Rome, do as the Romans do.
Semper Excelsius is a website for the defense of the Catholic faith through instruction and informative articles. Its focus is on explaining the rich history and traditions of the holy Roman Catholic Church, while defending her teachings against false assumptions and doctrinal errors.