If you have not yet read Part 1: Jack-O-Lantern, Stingy Jack, Jack the Smith, then now would be a good time to get caught up! *wink*
All Saints Day/All Hallows/Hallowmas/Lemuria/Samhain
Now, I'm not Catholic, but as explained to me by a friend, this day seems to be currently thought of as a day to remember loved ones who have died.
More specifically, it has been thought of as a day to commemorate those who have died and now reside in heaven.
Previous to that it was a day dedicated specifically to martyrs, but later came to be commemorative of all souls ("saints") who have gone to heaven, as stated above. Byzantine Emperor Leo VI "the Wise" supposedly built a church which he intended to dedicate to his deceased wife, Empress Theophano. When he was forbidden to dedicate it to her specifically, he decided to dedicate it to "All Saints," so that if his wife were in fact one of the righteous, she would also be honored whenever the feast of the Solemnity of All Saints was celebrated. According to Eastern tradition, it was Leo who expanded the feast from a commemoration of All Martyrs to a general commemoration of All Saints, whether martyrs or not.
The Wikipedia articles on these might give you a bit more insight if you'd like to look them up. Alot of my info is paraphrased and excerpted from there and other places. It's a bit confusing so I tried to compile it and simplify it a bit for clarity.
In Roman religion, the Lemuralia or Lemuria (the Feast of the Lemures) was a feast during which the ancient Romans performed rites to exorcise the malevolent and fearful ghosts of the dead from their homes. The unwholesome spectres of the restless dead, the lemures or larvae were appeased with offerings of beans. On those days, the Vestals would prepare sacred mola salsa (salt cake) from the first ears of wheat of the season. In the Julian calendar the three days of the feast were the 9th, 11th, and 13th of May. According to Ovid, at this festival it was the custom to appease or expel the evil spirits by walking barefoot and throwing black beans over the shoulder at night. It was the head of the household who was responsible for getting up at midnight and walking around the house with bare feet throwing out black beans and repeating the incantation, "I send these; with these beans I redeem me and mine (haec ego mitto; his redimo meque meosque fabis.)." nine times. The household would then clash bronze pots while repeating, "Ghosts of my fathers and ancestors, be gone!" nine times. According to cultural historians, this ancient custom was Christianized in the feast of All Saints' Day, established in Rome first on May 13, in order to de-paganize the Roman Lemuria. In the eighth century, as the popular observance of the Lemuria had faded over time, the feast of All Saints was shifted to November 1, coinciding with the similar Celtic harvest festival Samhain.
The Ulster Cycle (book of Irish mythology) is peppered with references to Samhain. Many of the adventures and campaigns undertaken by the characters therein begin at the Samhain Night feast. One such tale is Echtra Nerai ('The Adventure of Nera') concerning one Nera from Connacht who undergoes a test of bravery put forth by King Ailill. The prize is the king's own gold-hilted sword. The terms hold that a man must leave the warmth and safety of the hall and pass through the night to a gallows where two prisoners had been hanged the day before, tie a twig around one man's ankle, and return. Others had been thwarted by the demons and spirits that harassed them as they attempted the task, quickly coming back to Ailill's hall in shame. Nera goes on to complete the task and eventually infiltrates the sídhe (ghostly beings believed to be ancestors, spirits of nature, gods and godesses, or faeries) where he remains trapped until next Samhain. The sluagh sídhe - "the fairy host" are sometimes depicted in Irish and Scottish lore as airborne spirits of an unpleasant nature, and perhaps the cursed, evil or restless dead. The Banshee or bean sídhe, which simply means "woman of the sídhe", has come to specifically indicate the supernatural women of Ireland who announce an oncoming death by their wailing and keening. Her counterpart in Scottish mythology is the bean shìth. Other varieties of aos sí and daoine sìth include the Scottish bean nighe - the washerwoman who is seen washing the bloody clothing or armour of the person who is doomed to die.
"Thwarted by demons and spirits" eh? Banshees? Spirits appearing as portents of death? Gettin' a wee bit creeped out yet? It's not over. Here's more on the rituals of Samhain...
In parts of western Brittany, Samhain is still heralded by the baking of kornigou, cakes baked in the shape of antlers to commemorate the god of winter shedding his 'cuckold' horns as he returns to his kingdom in the Otherworld. The Romans identified Samhain with their own feast of the dead, the Lemuria. This, however, was observed in the days leading up to May 13. With Christianization, the festival in November (not the Roman festival in May) became All Hallows' Day on November 1 followed by All Souls' Day, on November 2. Over time, the night of October 31 came to be called All Hallow's Eve, and the remnants festival dedicated to the dead eventually morphed into the secular holiday known as Halloween.
Traditionally, Manx children dress as scary beings, carry turnips (ringing any bells from part 1?) and sing an Anglicized version of Jinnie the Witch. They go from house to house asking for sweets or money.
Celtic Reconstructionists place emphasis on historical accuracy, and base their celebrations and rituals on traditional lore from the living Celtic cultures, as well as research into the older beliefs of the polytheistic Celts. At bonfire rituals, some observe the old tradition of building two bonfires, which celebrants and livestock then walk or dance between as a ritual of purification.
According to Celtic lore, Samhain is a time when the boundaries between the world of the living and the world of the dead become thinner, allowing spirits and other supernatural entities to pass between the worlds to socialize with humans. It is the time of the year when ancestors and other departed souls are especially honored or feared.
Though Celtic Reconstructionists make offerings to the spirits at all times of the year, Samhain in particular is a time when more elaborate offerings are made to specific ancestors. Often a meal will be prepared of favorite foods of the family's and community's beloved dead, a place set for them at the table, and traditional songs, poetry and dances performed to entertain them. A door or window may be opened to the west and the beloved dead specifically invited to attend. Many leave a candle or other light burning in a western window to guide the dead home. Divination for the coming year is often done, whether in all solemnity or as games for the children. The more mystically inclined may also see this as a time for deeply communing with the deities, especially those whom the lore mentions as being particularly connected with this festival.
Divination games for the children? Inviting the dead into your home? Walking and driving livestock between fires to "purify?" Offerings of appeasement to spirits? Some creepy stuff there, eh?
Samhain is considered by some Wiccans as a time to celebrate the lives of those who have passed on, and it often involves paying respect to ancestors, family members, elders of the faith, friends, pets and other loved ones who have died. In some rituals the spirits of the departed are invited to attend the festivities. It is seen as a festival of darkness, which is balanced at the opposite point of the wheel by the spring festival of Beltane, which Wiccans celebrate as a festival of light and fertility.
So there you have the origins tracing back to before the holliday was "Christianized." And wow I can see why they thought it needed Christianized. Eek! Some nasty stuff in there, yes?
Part 3: How it all ties in, Christian response
Part 4: My Personal Beliefs