The custom to use ashes as a sign of repentance dates from biblical times, prior to Lord Jesus (2Sam 13:19; Job 42:5-6; Mt 11:21; Lk 10:13) and for this reason it has been chosen to be spread symbolically on the forehead of the penitent as a sign of sorrow and repentance on the Wednesday, six weeks before Easter.
This custom is very old in Western Christendom. The council of Nicaea proclaimed a period of 40 days fasting prior to the commemoration of the sacrifice of Lord Jesus on the Cross (1), and Roman Pope Gregory I (540-604CE) is said to have put ashes on ‘Ash Wednesday’ as initiation of this fasting period (2); finally, in the year 1091, Roman Pope Urban II decreed the custom to be official in Rome (3).
ASH WEDNESDAY TODAY
Even as this is not a biblical command, the custom has been passed on to all branches of Western Christianity (It has never been practiced in the East), to each church according to their tradition.
Traditional Churches, like Catholic, Western Orthodox, Anglican, and Lutheran, follow a strict procedure to make these ashes, and other less liturgical churches, like the Methodist and Presbyterian use common vegetable ash and apply it symbolically without a formal rite.
Then again, other modern Churches like Baptist and Pentecostals do not follow the tradition.
But the important thing, is that all of them, expect attentively the memorial of the holy Passion and Sacrifice of Lord Jesus six weeks after, in Easter, regardless of what day exactly they celebrate the crucifixion day.
This is a good start, ashes or not, to meditate for 40 days, like Lord Jesus did in the desert, over the great sacrifice and love that took to attain our salvation, and the fact that Jesus of Nazareth did it, willingly, out of his eternal love for us.
And He deserves all our worship, love, gratitude, and obedience, for all eternity.
(1) Gunther Gassmann, Mark W. Oldenburg, Historical Dictionary of Lutheranism, 2011. p. 229.
(2) Ted Olsen, "The Beginning of Lent". Christianity Today, 2008.
(3) John W. Fenton, "Orthodox Ash Wednesday". Antiochian Western Rite Vicariate, 2013.