Why ashes on Ash Wednesday?
Ash Wednesday emphasizes two themes: our sinfulness before God and our human mortality. The service focuses on both themes, helping us to realize that both have been triumphed through the death and resurrection of Jesus Christ.
During some Ash Wednesday services, the minister will lightly rub the sign of the cross with ashes onto the foreheads of worshipers. The use of ashes as a sign of mortality and repentance has a long history in Jewish and Christian worship. Historically, ashes signified purification and sorrow for sins.
It is traditional to save the palm branches from the previous Palm Sunday service to burn to produce ashes for this service. Sometimes a small card or piece of paper is distributed on which each person writes a sin or hurtful or unjust characteristic. The cards are then brought to the altar to be burned with the palm branches. The ash cross on the forehead is an outward sign of our sorrow and repentance for sins.
— Adapted from The United Methodist Book of Worship