The Celtic people started Samhain, the festival that eventually became known as Halloween, in the early days of the first millennium A.D. The festival celebrated the dead, whom the Celtics believed could join the living on that day.
The festival of Samhain coincided with Nov. 1. In the 700s, the Catholic church established Nov. 1 as the feast of All Saints, also known as All Hallows, in an effort to supplant the Samhain festival. This attempt was largely unsuccessful, as the Celtics continued the Samhain practices on the night before, All Hallows Eve. The name of the festival eventually became Halloween.