How did Halloween come into being?

Modern Halloween grew out of a number and ancient beliefs and practices, most notably the Gaelic Samhain and the Catholic All Saints Day. It is important to note that while Samhain dates back to before Roman times, it was Pope Gregory III who designated November first as All Saints Day.

The evening before All Saints Day became known as All Hallows' Eve and it gradually came to incorporate many of the traditions of Samhain. This holiday spread to North America primarily by Irish and Scottish immigrants. As time passed, for most people All Hallows' Eve began to lose its religious overtones and it became a more secular holiday.