The first Wednesday in Lent, so called from an ancient Roman
Catholic custom of sprinkling ashes on the heads of those condemned to
do penance on this day.
The ashes were those of the palms burnt on Palm Sunday. The
pessimi were sprinkled with ashes, the less offending were signed
on the forehead with the sign of the cross, the officiating minister
saying, “Memento, homo, quia pulvis es, et in pulverem reverteris.” The custom, it, is said, was introduced by Gregory the Great.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
