GIST ME JOOR

All Souls' Day is marked on 2nd November (or the 3rd if the 2nd is a Sunday), directly following All Saints' Day, and is an opportunity for Roman Catholics and Anglo-Catholic churches to commemorate the faithful departed.
They remember and pray for the souls of people who are in Purgatory - the place (or state) in which those who have died atone for their less grave sins before being granted the vision of God in Heaven (called Beatific vision).

Reasoning behind this stems from the notion that when a soul leaves the body, it is not entirely cleansed from venial (minor) sins. However, through the power of prayer and self-denial, the faithful left on earth may be able to help these souls gain the Beatific Vision they seek, bringing the soul eternal sublime happiness.

A 7/8th century AD prayer The Office of the Dead is read out in churches on All Souls' Day. Other rituals include the offering of Requiem Mass for the dead, visiting family graves and reflecting on lost loved ones. In Mexico, on el dia de los muertos (Day of the Dead), people take picnics to their family graves and leave food out for their dead relatives.

Whilst praying for the dead is an ancient Christian tradition, it was Odilo, Abbot of Cluny (France) who, in 998AD, designated a specific day for remembering and praying for those in the process of purification. This started as a local feast in his monasteries and gradually spread throughout the Catholic Church towards the end of the 10th century AD.