At the beginning of Lent, on Ash Wednesday, ashes are blessed during Mass, after the homily. The blessed ashes are then "imposed" on the faithful as a sign of conversion, penance, fasting and human mortality.
The act of putting on ashes symbolizes our fragility and mortality, and the need to be redeemed by the mercy of God. Far from being a merely external act, the Church has retained the use of ashes to symbolize that attitude of internal penance to which all the baptized are called during Lent.
From the very early times the commemoration of the approach of Christ's passion and death was observed by a period of self-denial. St. Athanasius in the year 339 enjoined upon the people of Alexandria the 40 days' fast he saw practiced in Rome and elsewhere, "to the end that while all the world is fasting, we who are in Egypt should not become a laughing stock as the only people who do not fast but take our pleasure in those days." On Ash Wednesday in the early days, the Pope went barefoot to St. Sabina's in Rome "to begin with holy fasts the exercises of Christian warfare, that as we do battle with the spirits of evil, we may be protected by the help of self-denial."