The Wednesday after Quinquagesima Sunday, which is the first day of the Lenten fast.
The name dies cinerum (day of ashes) which it bears in the Roman Missal is found in the earliest existing copies of the Gregorian Sacramentary and probably dates from at least the eighth century. On this day all the faithful according to ancient custom are exhorted to approach the altar before the beginning of Mass, and there the priest, dipping his thumb into ashes previously blessed, marks the forehead — or in case of clerics upon the place of the tonsure — of each the sign of the cross, saying the words: "Remember man that thou art dust and unto dust thou shalt return." The ashes used in this ceremony are made by burning the remains of the palms blessed on the Palm Sunday of the previous year. In the blessing of the ashes four prayers are used, all of them ancient.