The word itself, "Halloween," actually has its origins in the Catholic Church. It comes from a contracted corruption of All Hallows Eve. November 1, "All Hollows Day" (or "All Saints Day"), is a Catholic day of observance in honor of saints.
But, in the 5th century BC, in Celtic Ireland, summer officially ended on October 31. The holiday was called Samhain (sow-en),
The Gauls claim all to be descended from Father Dis [a god of death, darkness and the underworld], declaring that this is the tradition preserved by the Druids. For this reason they measure periods of time not by days but by nights; and in celebrating birthdays, the first of the month, and new year’s day, they go on the principle that the day begins at night.
Some other names are also associated with the Celtic New Year:
The Third Harvest
All Hollows Eve
The Day of the Dead
And the famous Halloweens Day.
The tradition of trick-or-treating dates back to the early All Soul’s Day in England, during which poor citizens would beg for food and families would give them pastries called “soul cakes” in return for their promise to pray for the family’s dead relatives. The distribution of soul cakes was encouraged by the church as a way to replace the ancient practice of leaving food and wine for roaming spirits
Have a very scary Halloween