All Souls' Day is a Holy Day set aside for honoring the dead. It is a day of prayer and remembrance for those souls in Purgatory (the day prior for being all those in Heaven) and is known in the Liturgical books as the Commemoration of All the Faithful Departed (Commemoratio omnium fidelium defunctorum).
Saint Isidore of Seville wrote that the Monday after Pentecost was a day designated to remember the deceased and Abbot Eigil of Fulda set December 17th as a day of remembrance for their souls in Germany sometime in the early 9th Century. Saint Odilo of Cluny chose the day immediately following All Saints' Day as an annual commemoration of the departed and ordered that the day be observed with alms, prayers and sacrifices to help all souls in purgatory. This date and custom spread quickly throughout the Church, especially in the Benedictine monasteries. In 1008, Bishop Notger adopted the practice and date officially for the Diocese of Liege.
Beginning in the 15th century, the Dominicans instituted the practice of each priest offering three Masses on the Feast of All Soups. Pope Benedict XV granted all priests the privilege and ability to offer three Masses on All Souls' Day due to the uncountable number of dead and destroyed Churches in World War 1. Priests celebrating Mass wear vestments of varying colors - Black for morning, Violet for penance, or White for the hope of Resurrection through Christ.
In Tirol (part of Austria), cakes are left on the table for souls and the room kept warm for their comfort. In Brittany, people kneel at the graves of loved ones to anoint the hollow of the tombstone with Holy Water. A traditional dinner in Malta has a roasted pig - the big is first let loose on the street with a bell around it's neck before being cooked and fed to the poor. All Souls' Day is the second day of Dia de Muertos in Mexico.
A full indulgence can be gained each day from Nov 1 to Nov 8 to pray for the department and fulfilled other conditions (such as going to Mass on All Soul's Day). In 2020, due to COVID, the full indulgence period was extended to encompass all of November.