But why is the evening of October 31st holy? 

Modern day Halloween has, as a lot of people know, very old roots. It is believed to have originated as the feast Samhain. 

The Celtic New Year began on November 1st, and this was marked by the feast Samhain. At this time of year, the days are visibly getting shorter and the seasons are changing. It was believed that on the night between October 31st and November 1st (and therefore between the old and the new year), the veil between the land of the living and the dead was thinner than normal. The spirits of all who died the previous year were believed to be journeying to the otherworld, and the souls of the dead would be allowed to return to visit their homes.

People lit huge bonfires to scare away evil spirits, and they sometimes wore masks to avoid being recognized by malicious ghosts. 

Since the veil between worlds was thinner than normal, Samhain was also a great time for divination and predictions about the future. These prophecies made by druids were an important direction for the Celtic people during the long winter that followed.

However, the druids weren't the only ones who believed the veil between worlds was thinner at that time of year; it is a belief that was shared by other cultures as well.

When the Roman Empire conquered great parts of the Celtic territory, two Roman festivals were merged with Samhain: Feralia, a day for commemorating the dead, and a day to honour Pomona, the Roman goddess of fruit and trees. A lot of modern traditions can be linked with this merged festival.

Originally, All Saints' Day was known as All Martyrs’ Day. It was created by Pope Boniface IV in the 7th century and fell on May 13th. In the 8th century, it was moved by Pope Gregory III to fall on November 1st as it is today, and the festival was expanded to include all saints instead of just martyrs. Around 1000 AD (after Christianity had spread to the Celtic lands), All Souls’ Day was added in what is widely believed to be an attempt to replace the pagan feast of Samhain. However, All Souls’ Day was celebrated much like Samhain.

Traditions like lantern carving that was originally meant to scare away evil spirits still continue today, but the original intention has vanished and been replaced by tradition. 

However, many new traditions have been added, and Halloween is now a big mix between old and new, and it is a festivity loved by many.