All Saints Day - 1 November
In the year 835 AD the Roman Catholic Church made 1st November a church holiday to honour all the saints. Although it was a joyous holiday it was also the eve of All Souls Day, so in Medieval times it became customary to pray for the dead on this date.
Another name for All Saints Day is 'All Hallows' (hallow is an archaic English word for 'saint'). The festival began on All Hallows Eve, the last night of October.