Samhain is the Celtic Feast of the Dead and is one of the 8 sabbats in the pagan calendar. Samhain may be pronounced in a number of ways but the most common pronunciation is "sow-in" (sow rhymes with cow). The modern day Halloween celebration is a descendent of this ancient festival.
The Celtic celebration signaled the end of the harvest (fruits, nuts, and souls) and the beginning of winter. The Celts believed that, during Samhain, "the barriers between humans, ancestors, gods and faeries are overthrown, and they can visit each others' realms".
Other divination practices occurred during Samhain and some of these followed Irish immigrants to America during the potato famine. For example, bobbing for apples was a marriage divination that indicated who (the first person to bite an apple) would marry first in the coming year.
Samhain is celebrated in modern times by many members of pagan religions, including Druids and Wiccans. It is observed as a "memorial day for their dead friends, similar to the national holiday of Memorial Day in May...a time to wrap up old projects, take stock of ones life, and initiate new projects for the coming year...and a time to begin hand work such as sewing, leather working, wood working, etc. for Yule gifts later in the year".
1. Matthews, C. (1996). The Celtic Book of Days. Rochester, VT: Destiny Books.