In the Western Christian calendar, Ash Wednesday is the first day of Lent and occurs forty days before Easter (excluding Sundays).

At Masses and services of worship on this day, ashes are imposed on the foreheads (or tonsure spots, in the case of some clergy) of the faithful. The priest, minister, or in some cases officiating layperson marks the forehead of each participant with black ashes in the shape of a cross, which the worshiper traditionally retains until washing it off after sundown.

The ashes used in the service of worship or Mass are sacramentals, not a sacrament. The ashes may be prepared by burning palm leaves from the previous year's Palm Sunday celebrations. They are blessed according to various rites proper to each liturgical tradition, sometimes involving the use of Holy Water. In some churches they are mixed with light amounts of water or olive oil, which serve as a fixative.