The celebration of Halloween has its roots in the pre-Christian, Celtic festival of Samhain. During this time of year, it was believed, the nearly deceased were able to travel to the world beyond, while other spirits could return to their earthly home. Fires were lit to ward of evil spirits. During the 7th century, Pope Boniface IV established All Saints’ Day during the month of May, though this was later moved to November 1. The evening before All Saints’ Day became known as hallowed eve, or holy eve, and, later, as Halloween.
Our modern celebration is largely due to the large influx of Irish immigrants into the USA during the 19th century. They brought their festivals with them, and this included Halloween. And it was in the USA, consequently, that the ancient pagan festival was made popular again (albeit in a modern and more secular form). Nevertheless, with the growth of neopaganism (such as Wicca and Druidry), and the return to older, pre-Christian festivals, followers of the “old ways” will, of course, be celebrating Samhain this evening.
Here are a couple of videos on the history of Halloween/Samhain, the first by Survive The Jive and the second by Arith Harger: