This season has 3 holidays that follow one after the other, starting with Halloween on October 31st, and ending with All Souls Day on November 2nd. In the U.S. Halloween is the most famous of the 3 days. The name Halloween means All Hallows Eve because it is the evening or night before All Hollows, or as it is now called, All Saints Day. In Scotland, the word for evening is e'en which, when preceded by hallow, gives us hallowe'en or Halloween.
The ancient Celtic religious festival of Samhain (pronounced sow-in) began on October 31st. This festival marked the end of summer, the end of harvest and the beginning of winter with its long dark nights and short days. Because the Celts, like many ancient peoples, measured the day as going from sunset to sunset, the Samhain holiday began at sunset on October 31st.
Samhain marked a turning point in the Celtic year and as the world symbolically transitioned from summer to winter on this day, the normal rules of daily life were relaxed. It was also a day when the barrier between our world and the spirit world was temporarily opened, allowing spirits easy access to the physical world. With spirits of the dead abroad in the world on the night Hallowe'en, people started dressing in costumes to both scare the spirits away and to be unrecognized by the dead spirits. The Jack O' Lantern evolved from an Irish tale about a man named Stingy Jack who managed to anger both God and Satan and was barred from both Heaven and Hell upon his death. When Jack attempted to enter Hell after his death, the Devil turned him away and condemed him to wander the earth at night with only a burning coal to light his way. Jack carried the coal in a hollow turnip. When the Irish came to America in the 19th century, they switched from turnips to pumpkins to make Jack O' Lanterns.
Like Christmas, Halloween began as a pagan festival that later converted to a Christian holiday and in recent times evolved into a combination secular and Christian holiday. But despite the presence of assorted spirits, including devils, in the Halloween tradition, Halloween has never been historically associated with devil worship. The ancient Celtic religion did not include evil satanic worship and at no point in it's history has devil worship played a part in the Halloween tradition. Any connections to devil worship are modern attempts by people of this persuasion to take over this holiday.
The second of the three days is All Saints Day. This is the day that the Church has set aside to honor all the saints in the Church both known and unknown.
In the early Church it was common for local churches to celebrate the anniversary of the martyrdom of their members who had died for their faith. As early as 411 A.D. many churches in what is now the Middle East began Designating the first Friday after Easter as the general feast day for all martyrs.
In the 8th century, Pope Gregory III (731-741) consecrated a chapel in St. Peter's Basilica to all the saints (honoring all those who died and gone to Heaven, not just those who had been martyred) and established November 1st as the day of the feast. This coincided with the Celtic festival of Samhain and gave the festival a more Christian focus thereby absorbing it into the Christian tradition.
Today, All Saints Day is a major feast day in the Roman Catholic Church and some other Christian churches in the West and is celebrated on November 1st. In the Orthodox churches in the East, All Saints Day is observed as the first Sunday after Pentecost, thereby remaining in May.
The third day is All Souls Day, another religious holiday. This is the day the Catholic Church sets aside to pray for the deceased who are in Purgatory awaiting entry into Heaven. It is celebrated on November 2nd immediately following All Saints Day. Odilo, abbot of clunny in the eleventh century is generally credited with establishing All Souls Day as a common feast day for the Church as a whole.
In Mexico (and now parts of the southwestern U.S. with a large Mexican population) All Souls Day is known as Dia de los Muertos or "Day of the Dead". The date tends to vary some by area, but Dia de los Muertos is observed as the same time as Halloween, All Saints Day and All Souls Day.
Dia de los Muertos is the old Aztec holiday set aside to honor the dead. When the Spanish arrived in Mexico and Central America, they tried to eradicate the holiday as part of their efforts to convert the local population to Christianity. But when that failed, they moved the holiday to coincide with All Souls Day and changed the focus from pagan to Christian while keeping many of the traditions and customs of the old Aztec holiday.