The Old-English halig developed into the adjective holy as well as into the noun hallow. So a hallow is a holy person, a Saint.
Hallow-e’en is a shortened form of All-Hallow-Even ‘All Saints’ Day Eve’, the eve of All Saints’ Day on November 1. In former days one didn’t count time in days, but in nights. When counting in nights, the evening is not seen as the end of the day, but as the beginning of the night. Hence e.g. the celebration of Christmas Eve.
The Church replaced pagan feasts with christian ones, but many pagan customs remained. In the old Celtic calendar the year began on November 1. The evening of October 31 therefore was New Year’s Eve, the night of all ghosts and witches. The Church transformed it into All Saints’ Day Eve.
The feast has been brought to the USA by English and even more by Irish immigrants. With the current American influence it is gaining popularity on the European continent.
Would you mind sharing it with us?
I think the history of trick or treating is that a long time ago, some witches went to these peoples house, and said, “Give us your children! We want to EAT THEM!!!” But the people in the house didn’t want to give their kids away. So the people in the house gave them gingerbread men cookies and said, “Here, we have these gingerbread children. Take them instead!” So the witches took them and they were all like, “Yummy!” So the witches went around to peoples houses to get cookies from them.
I saw that on The Simpsons, so you know it has to be true.