The origins of All Souls' Day (Day of the Dead) in European folklore and folk belief are related to customs of ancestor veneration practiced worldwide, such as the Chinese Ghost Festival or the Latin American Day of the Dead. The Roman custom was that of the Lemuria.
theological basis for the feast is the doctrine that the souls which, on
departing from the body, are not perfectly cleansed from venial sins, or have
not fully atoned for past transgressions, are debarred from the Beatific Vision,
and that the faithful on earth can help them by prayers, almsdeeds and especially
by the sacrifice of the Mass