Ash Wednesday marks the beginning of the Lenten season, which lasts until the Easter Vigil. On Ash Wednesday and Good Friday, Catholics between the ages of 18 and 59 are permitted to consume only one full meal each day, which may be supplemented by two smaller meals, which together should not equal the full meal. These days are also days of abstinence from meat.

At Masses and Services of worship on this day, worshippers are blessed with ashes by the celebrating priest or minister. The priest or minister marks the forehead of each participant with black ashes, in the shape of a cross, which the worshipper traditionally retains until washing it off after sundown. The symbolism echoes the ancient Near Eastern tradition of throwing ash over one's head signifying repentance before God (as related numerous times in the Bible). The priest or minister offers the worshipper an instruction while applying the ashes.

The ashes are prepared by burning palm leaves from the previous year's Palm Sunday celebrations and mixing them with olive oil as a fixative. In the Roman Catholic Church, Ash Wednesday is observed by fasting, abstinence (from meat), and repentance—a day of contemplating one's transgressions. The ashes are sacramentals, not a sacrament. The penitential psalms are read.

The Anglican Book of Common Prayer designates Ash Wednesday as a day of fasting.

As the first day of Lent, it comes the day after Shrove Tuesday or Mardi Gras, the last day of the Carnival season. The word "Carnival" is in fact derived from the Italian word carne "meat", in reference to the Lenten practice of giving up meat.