Our word Halloween is derived from the old words All Hallows Even (All Hallows Evening) because it was the eve of All Hallows Day (I November). Hallow is an old word for saint and today we call 1 November All Saints Day.
In the 4th century the Church began to celebrate a feast to all the martyrs. At first it was celebrated on the first Sunday after Pentecost (in late May or early June). The Eastern Orthodox Churches still celebrate it on that day. However in the West in 609 or 610 Pope Boniface IV moved the feast of all martyrs to 13 May. Then in the 8th century Pope Gregory III (731-741) made 1 November a feast to remember all saints and it became known as All Saints Day or All Hallows Day.
After the 16th Century Reformation All Saints Day and Halloween were virtually forgotten in most of England, along with most other saints days. However Halloween continued to be observed in Scotland and Ireland. There was a belief that ghosts and witches were particularly active on that day.