The Philocalian calendar of AD 354 gives a festival of "Natalis Invicti" on 25 December. There is limited evidence that this festival was celebrated at around this time before the mid-4th century. The charioteer in the mosaic of Mausoleum M has been interpreted by some as Christ. Clement of Alexandria had spoken of Christ driving his chariot across the sky. This interpretation is doubted by others: "Only the "cross-shaped" nimbus makes the Christian significance apparent". and the figure is seen by some simply as a representation of the sun with no explicit religious reference whatever, pagan or Christian.