A study of Augustine of Hippo remarks that his exhortation in a Christmas sermon, "Let us celebrate this day as a feast not for the sake of this sun, which is beheld by believers as much as by ourselves, but for the sake of him who created the sun", shows that he was aware of the coincidence of the celebration of Christmas and the Birthday of the Unconquered Sun, although this pagan festival was celebrated at only a few places and was originally a peculiarity of the Roman city calendar. It adds: "He also believes, however, that there is a reliable tradition which gives 25 December as the actual date of the birth of our Lord."