All Saints’ Day is officially the Solemnity of All Saints. This is a Holy Day of Obligation and is celebrated on November 1. In Western Christian theology, the day commemorates all those who have attained the beatific vision in Heaven. All Saints’ Day is a national holiday in many historically Catholic countries.
In the Roman Catholic Church, the next day, November 2, All Souls’ Day, which is not a Holy Day of Obligation, specifically commemorates the departed faithful who have not yet been purified and reached heaven. The 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.
Catholics celebrate All Saints’ Day and All Souls’ Day in the fundamental belief that there is a prayerful spiritual communion between those in the state of grace who have died and are either being purified in purgatory or are in heaven (the ‘church penitent’ and the ‘church triumphant’, respectively), and the ‘church militant’ who are the living, the three groups which constitute the Communion of Saints. (9:49)