Halloween is seen as a secular holiday, and some of the more fundamental religions outright ban it. While to most people, October 31st is a day for scary fun, parties and costumes, the extremely orthodox of many religions see it as a night of evil or mischief. Who's right? Well, neither, exactly.
Halloween began as a religious holiday - not an evil one - and gradually morphed into the frenzied sugar- and alcohol-fueled street party we know today. The origins of the day began as something very serious and solemn, but became a time of revelry as the old religions fell by the wayside and new customs were adopted.
Samhain was the original Halloween, a pagan holiday in late October that simultaneously marked the end of the harvest, the coming of winter and a chance to honor ancestors. The Celts believed that the separation between the living and the dead became very thin on Samhain, and took the opportunity to communicate with deceased loved ones and honor their memories.
The Druids believed that having spirits around facilitated predictions, so they performed elaborate rituals and looked into the future to better prepare for the coming winter. The night commenced with the ritual lighting of the bonfire, which was symbolic of comfort and protection from the cold.
When the Romans conquered much of the Celtic territory, Samhain became fused with Feralia, the Roman holiday that also celebrated the dead, which also occurred in late October. In 609 AD, the pope dedicated the Pantheon to Catholic martyrs, thus establishing All Martyrs' Day, which was expanded to All Saints' Day within a hundred years. All Saints' Day was celebrated on November 1st, and November 2nd was declared All Souls' Day in an effort to replace the pagan holiday with a church-approved event.
This is when our modern notion of Halloween began. By 1000 AD, the holidays of Samhain, All Saints' Day and All Souls' Day became enmeshed into a single celebration, complete with bonfires, apples (a symbol of the Roman goddess of fruit), and costumes of angels and devils and saints. The Middle English translation of All Saints' Day is Alholowmesse, referring to the November 1st holiday. Certain circles still practiced the pagan Samhain the night before calling it All-Hallows Eve, which eventually became Halloween.
Halloween wasn't exactly popular in the new America, and was limited to the southern colonies with lower concentrations of Protestants - in fact, it wasn't exactly Halloween at all. The holiday reverted back to a harvest celebration, albeit one with ghost stories and mischief - it seems the early American south unknowingly resurrected Samhain - but the costumes, bonfires and general revelry did not cross the Atlantic.
The great immigrant influx of the 1800s brought more Halloween customs, and Americans began trick-or-treating, dressing in costumes and asking neighbors for food or money. The belief in Halloween future-telling came back as well, with young women performing elaborate spells to divine the name of the man they would marry.
There was a push in the late nineteenth century to make Halloween more of a community holiday than one associated with harvests or spirits - in short, pretty much the Halloween we know today. Parties for the whole family were common, and centered around games and seasonal food. Truly scary costumes were discouraged out of concern for the children.
By the mid-twentieth century, Halloween pranking got so out of hand that many towns were forced to severely limit celebrations or cancel them altogether - a move that steered the holiday toward children, rather than grownup celebration. That particular custom has fallen by the wayside since the late 20th century, as adults realize they enjoy candy, costumes and parties just as much as the kids - so much, in fact, that Halloween has become a multi-billion dollar industry, the second biggest American holiday.