In the western church the first day of Lent is called Ash Wednesday from the ceremonial use of ashes, as a symbol of penitence, in the service prescribed for the day. The custom is still retained in the Roman Catholic Church as well as the Anglican, Episcopal and Lutheran Churches. The ashes, obtained by burning the remains of the palm branches blessed on the previous Palm Sunday, are placed in a vessel on the altar and consecrated before High Mass. The priest then invites those present to approach and, dipping his thumb in the ashes, marks them as they kneel with the sign of the cross on the forehead, with the words: Remember, man, thou art dust and unto dust thou shalt return.
This ceremony is derived from the custom of public penance in the early church. When the custom was extended to the entire congregation is not known, although it seems to have been in common use by the late 10th century.
Bill Petro, your friendly neighborhood historian