All Souls' Day commemorates the faithful departed. In Western Christianity, this day is observed principally in the Catholic Church, although some churches of the Anglican Communion and the Old Catholic Churches also celebrate it.
The Eastern Orthodox churches observe several All Souls' Days during the year. The Roman Catholic celebration is associated with the doctrine that the souls of the faithful who at death have not been cleansed from the temporal punishment due to venial sins and from attachment to mortal sins cannot immediately attain the beatific vision in heaven, and that they may be helped to do so by prayer and by the sacrifice of the Mass. In other words, when they died, they had not yet attained full sanctification and moral perfection, a requirement for entrance into Heaven. This sanctification is carried out posthumously in Purgatory.
The official name of the celebration in the Roman Rite liturgy of the Roman Catholic Church is "The Commemoration of All the Faithful Departed". Another popular name in English is Feast of All Souls. In some other languages the celebration, not necessarily on the same date, is known as Day of the Dead.
The Western celebration of All Souls' Day is on 2 November and follows All Saints' Day, which commemorates the departed who have attained the beatific vision. If 2 November falls on a Sunday, the Mass is of All Souls, but the Office is that of the Sunday. However, Morning and Evening Prayer (Lauds and Vespers) for the Dead, in which the people participate, may be said. In pre-1969 calendars, which some still follow, and in the Anglican Communion, All Souls Day is instead transferred, whenever 2 November falls on a Sunday, to the next day, 3 November, as in 2008.
The Eastern Orthodox Church dedicates several days throughout the year to the dead, mostly on Saturdays, because of Jesus' resting in the tomb on Saturday. (From: Wikipedia)