Cardinal Sean P. O’Malley will celebrate Ash Wednesday Mass at St. Brigid Parish tonight in South Boston.

O’Malley, the archbishop of Boston, will join the Rev. Robert Casey, young adults, and parishioners at the church at 845 East Broadway at 6 p.m., an archdiocese spokeswoman said.

Ash Wednesday marks the first day of Lent, a 40-day liturgical period of prayer and fasting in the Catholic religion.

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The day’s name is derived from the tradition of burning palm fronds — often those from the previous year’s Palm Sunday. The resulting ashes are then used to mark the sign of the cross on worshipers’ foreheads.