Halloween is just an old-fashioned way of saying “the night before All Saints’ Day” — also called Hallowmas or All Hallows’ Day.
This comes from the fact November 1 is All Saints’ Day, a Christian feast dedicated to celebrating the faithful departed, including all the saints. In Christian tradition, people start celebrating major feasts the night before they take place — take Christmas Eve, for instance.
https://www.businessinsider.com/history-of-halloween-2017-10