Halloween has evolved and been influenced by a number of different cultures and religions, the most important of which are paganism, the Romans, the Celts (the people of Ireland, Scotland, Britain, Wales) and Christianity.
However, almost all of the traditions surrounding Halloween as we know it today can be traced back to the Celtic Day of the Dead, a pagan holiday.
Halloween originally comes from Ireland, where there was a group of people called the Celts.. When October ended and November began, the Celts held a festival to mark the end of summer and the gathering of their harvest for winter.
Samhain was the name of the Druid god of the dead. The Druids were a religious order amongst the Celts. On this day, they would try to appease their Lord of Death. These Druids also believed that witches rode on broom sticks and that ghosts were the cause of supernatural occurrences.
The belief was that on the eve of the Celtic New Year (which for them was October 31), the souls of the dead people roamed the land of the living. The Devil, spirits and witches were also believed to be moving about and at the height of their power.
On the night of October 31st, the Celts believed that ghosts would come around and haunt people for the night. In order to scare the ghosts away, it became tradition to dress up like other things that would fool the ghosts and bad spirits!
Later, when the region occupied by the Celts was conquered by the Romans, two Roman festivals (that respectively commemorated the passing of the dead and honored the goddess of the harvest) were combined with the Celtic festival of Samhain. Then, in the seventh century, Pope Boniface IV declared that November 1st would be All Saints’ Day, also known as All-hallows Day. While many people think he did this to try and change the focus of the preceding night’s festival, it is also where modern-day Halloween gets it name: The night before All-hallows Day became known as All-hallow’s Eve, and eventually “Halloween”!
Over time, as observances and traditions changed, Halloween went from being a Celtic harvest festival to the trick-or-treating holiday we know today! Lots of people still celebrate Halloween by dressing up in costume, and the holiday has moved from its original region and is now celebrated in many countries all over the world! While some people chose not to celebrate it because it is not a holiday that comes from their religion or culture, others have adapted it and celebrate anyway!
This explanation comes from reading Whyzz and Soundvision