How did it come to pass the celebration of halloween?

This is a bit of reading, but it is super interesting. I got this from the History Channels website.

“Halloween’s origins date back to the ancient Celtic festival of Samhain (pronounced sow-in).

The Celts, who lived 2,000 years ago in the area that is now Ireland, the United Kingdom, and northern France, celebrated their new year on November 1. This day marked the end of summer and the harvest and the beginning of the dark, cold winter, a time of year that was often associated with human death. Celts believed that on the night before the new year, the boundary between the worlds of the living and the dead became blurred. On the night of October 31, they celebrated Samhain, when it was believed that the ghosts of the dead returned to earth. In addition to causing trouble and damaging crops, Celts thought that the presence of the otherworldly spirits made it easier for the Druids, or Celtic priests, to make predictions about the future. For a people entirely dependent on the volatile natural world, these prophecies were an important source of comfort and direction during the long, dark winter.

To commemorate the event, Druids built huge sacred bonfires, where the people gathered to burn crops and animals as sacrifices to the Celtic deities.

During the celebration, the Celts wore costumes, typically consisting of animal heads and skins, and attempted to tell each other’s fortunes. When the celebration was over, they re-lit their hearth fires, which they had extinguished earlier that evening, from the sacred bonfire to help protect them during the coming winter.

By A.D. 43, Romans had conquered the majority of Celtic territory. In the course of the four hundred years that they ruled the Celtic lands, two festivals of Roman origin were combined with the traditional Celtic celebration of Samhain.

The first was Feralia, a day in late October when the Romans traditionally commemorated the passing of the dead. The second was a day to honor Pomona, the Roman goddess of fruit and trees. The symbol of Pomona is the apple and the incorporation of this celebration into Samhain probably explains the tradition of “bobbing” for apples that is practiced today on Halloween.

By the 800s, the influence of Christianity had spread into Celtic lands. In the seventh century, Pope Boniface I’ve designated November 1 All Saints’ Day, a time to honor saints and martyrs. It is widely believed today that the pope was attempting to replace the Celtic festival of the dead with a related, but church-sanctioned holiday. The celebration was also called All-hallows or All-hallowmas (from Middle English Alholowmesse meaning All Saints’ Day) and the night before it, the night of Samhain, began to be called All-hallows Eve and, eventually, Halloween. Even later, in A.D. 1000, the church would make November 2 All Souls’ Day, a day to honor the dead. It was celebrated similarly to Samhain, with big bonfires, parades, and dressing up in costumes as saints, angels, and devils. Together, the three celebrations, the eve of All Saints’, All Saints’, and All Souls’, were called Hallowmas.”

Halloween started as a pagan holiday where it was believed the souls of the dead could walk the earth. Dressing up was meant to scare them away.

Halloween has its origins in the ancient Celtic festival known as Samhain (Irish pronunciation: [ˈsˠaunʲ]; from the Old Irish samain). The festival of Samhain is a celebration of the end of the harvest season in Gaelic culture, and is sometimes regarded as the “Celtic New Year”. Traditionally, the festival was a time used by the ancient pagans to take stock of supplies and slaughter livestock for winter stores. The ancient Gaels believed that on October 31, now known as Halloween, the boundary between the alive and the deceased dissolved, and the dead become dangerous for the living by causing problems such as sickness or damaged crops. The festivals would frequently involve bonfires, into which bones of slaughtered livestock were thrown. Costumes and masks were also worn at the festivals in an attempt to mimic the evil spirits or placate them.