0 in Western Christianity, 1 November, when all the Christian saints (= people who have received an official honour for having lived in a good and holy way) are remembered. In the Eastern Orthodox Church, All Saints' Day is in the spring: --
Halloween trick-or-treating comes from the medieval Christian custom of begging for sweets on All Saints Day, November 1.
New Orleans also has a custom of having family reunions at the tomb on All Saints Day.
Some Catholic schools will be closed in honor of All Saints' Day, which is tomorrow.