All Saints' Day (also called All Hallows or Hallowmas), often shortened to All Saints, is a feast celebrated on November 1 in Western Christianity, and on the first Sunday after Pentecost in Eastern Christianity in honor of all the saints, known and unknown.

In terms of Western Christian theology, the feast commemorates all those who have attained the beatific vision in heaven. Specifically, in the Roman Catholic Church, the next day, All Souls' Day, commemorates the departed faithful who have not yet been purified and reached heaven.

Source: Wikepedia