In the Roman Catholic Church, Ash Wednesday is the first day of Lent, the season of preparation for the resurrection of Jesus Christ on Easter Sunday.
While Ash Wednesday is not a Holy Day of Obligation, all Roman Catholics are encouraged to attend Mass on this day in order to mark the beginning of the Lenten season.
The Distribution of Ashes:
During Mass, the ashes which give Ash Wednesday its name are distributed. The ashes are made by burning the blessed palms that were distributed the previous year on Palm Sunday; many churches ask their parishioners to return any palms that they took home so that they can be burned.
After the priest blesses the ashes and sprinkles them with holy water, the faithful come forward to receive them. The priest dips his right thumb in the ashes and, making the Sign of the Cross on each person's forehead, says, "Remember, man, that thou art dust, and to dust thou shalt return" (or a variation on those words).
A Day of Repentance:
The distribution of ashes reminds us of our own mortality and calls us to repentance. In the early Church, Ash Wednesday was the day on which those who had sinned, and who wished to be readmitted to the Church, would begin their public penance. The ashes that we receive are a reminder of our own sinfulness, and many Catholics leave them on their foreheads all day as a sign of humility.
Fasting and Abstinence Are Required:
The Church emphasizes the penitential nature of Ash Wednesday by calling us to fast and abstain from meat. Catholics who are over the age of 18 and under the age of 60 are required to fast, which means that they can eat only one complete meal and two smaller ones during the day, with no food in between. Catholics who are over the age of 14 are required to refrain from eating any meat, or any food made with meat, on Ash Wednesday and for every Friday until the Easter.
Taking Stock of Our Spiritual Life:
This fasting and abstinence is not simply a form of penance, however; it is also a call for us to take stock of our spiritual lives. As Lent begins, we should set out specific spiritual goals we would like to reach before Easter and decide how we will pursue them—for instance, by going to daily Mass when we can and receiving the Sacrament of Confession more often.
The Bible does not mention Ash Wednesday or the custom of Lent, however, the practice of repentance and mourning in ashes is found in 2 Samuel 13:19; Esther 4:1; Job 2:8; Daniel 9:3; and Matthew 11:21.(Source taken from http://catholicism.about.com/od/holydaysandholidays/p/Ash_Wednesday.htm)