All Saints Day and All Souls Day
From the early centuries of Christianity, Nov. 1 and 2 have been observed as days to remember the dead. This remembrance most likely originated in Celtic lands where this time of year marks the beginning of death in nature. By the 5th century CE in Rome, there was a festival for all the saints celebrated in the Pantheon, a Roman temple dedicated to all the gods.
Eventually Nov. 1 was marked as a day to remind the Christian faithful that they are in communion with all who have gone before them especially those who have lived holy lives but who are not canonized as saints. From this, Nov. 2 became a day more focused on all the dead, hence called All Souls Day.
All over Europe and in South America these two days are celebrated with visits to the cemeteries and by special commemorations in church to remember the dead. All Saints Day is marked by the Catholic Church as a special feast day. In many churches, it is customary to have a "book of remembrance" in which people write the names of their loved ones to be prayed for especially all during November.
Halloween (All Hallows Eve, the night before All Saints Day) in America is a popular spin-off from these ancient days of prayer.