Tonight is the night that the veil between the netherworlds is the thinnest.
According to Wikipedia:
The ancient Celts believed that the border between this world and the Otherworld became thin on Samhain, allowing spirits (both harmless and harmful) to pass through. The family’s ancestors were honoured and invited home while harmful spirits were warded off. It is believed that the need to ward off harmful spirits led to the wearing of costumes and masks. Their purpose was to disguise oneself as a harmful spirit and thus avoid harm. In Scotland the spirits were impersonated by young men dressed in white with masked, veiled or blackened faces. Samhain was also a time to take stock of food supplies and slaughter livestock for winter stores. Bonfires played a large part in the festivities. All other fires were doused and each home lit their hearth from the bonfire. The bones of slaughtered livestock were cast into its flames. Sometimes two bonfires would be built side-by-side, and people and their livestock would walk between them as a cleansing ritual.
The holiday itself is a combination of the Celtic Samhaim and the Christian All Saints Day. Today Halloween is a huge retail boon, and only Christmas surpasses it. Costumes and Candy are what bring in the money. Legend has it that vamps and other undead don’t mess with people on Halloween. Perhaps they just see it as amateur night.