Why are ashes placed on Christians’ foreheads on Ash Wednesday?

On Ash Wednesday, the application of ashes serves as a solemn reminder of human mortality and the necessity of reconciliation with God.1

Historically, the practice originated in Rome, where penitents and grievous sinners began a period of public penance by being sprinkled with ashes and wearing sackcloth to prepare for their restoration to the Eucharist. As these specific public penance rituals faded between the 8th and 10th centuries, the custom evolved into the current practice of placing ashes on the foreheads of the entire congregation to mark the beginning of the Lenten season.1

Today, the ashes—often created by burning the palms from the previous year’s Palm Sunday—are applied in the shape of a cross. They function as a memento mori, frequently accompanied by the reminder: "Remember that you are dust, and to dust you shall return".1