Short answers, pulled from the story.
Halloween, also known as All Hallows' Eve or All Saints' Eve, is celebrated on the 31st of October, the eve of the Western Christian feast of All Hallows' Day. It opens Allhallowtide, the period dedicated to remembering the dead, including saints, martyrs, and all the faithful departed.
The word Halloween comes from the Lowland Scots form of All Hallows' Eve, where even, the Scots term for evening, was contracted to e'en, turning All Hallow Even into Halloween. Hallow derives from the Old English halig, meaning holy, so the name means Saints' evening.
Many Halloween customs are thought to have been influenced by the Gaelic festival Samhain, celebrated from the 31st of October to the 1st of November in Ireland, Scotland, and the Isle of Man. Samhain marked the end of harvest and was seen as a liminal time when the boundary between this world and the Otherworld thinned.
Trick-or-treating has been linked to the medieval custom of souling, in which poor people, often children, went door to door during Allhallowtide collecting soul cakes in exchange for praying for the dead. The earliest known print use of the term trick or treat appears in 1917 in The Sault Daily Star of Sault Ste. Marie, Ontario.
Jack-o'-lanterns are tied to an Irish folktale about Jack, who tricked the Devil and was later refused entry to both heaven and hell, placing a live coal in a hollowed-out turnip to light his endless wandering. In Ireland, Scotland, and Northern England the turnip was traditionally carved, but North American immigrants used the native pumpkin, which was softer and larger and easier to carve.
Halloween accounts for over 10 billion dollars every year in the American economy. The National Retail Federation projected spending of 12.2 billion dollars in 2023, up from 10.6 billion in 2022, and forecasts an increase to 13.1 billion dollars in 2025.
Halloween became a major American holiday after mass Irish and Scottish immigration in the 19th century, having earlier been opposed by the Puritans of New England. Through American influence, its customs later spread to many other countries by the late 20th and early 21st centuries.