All Hallows' Eve, 31 October

All Hallows' Eve falls on the thirty first of October, the day this rule fixes on the Gregorian calendar, because it is the vigil of All Saints on the first of November, the older and doctrinally primary feast the evening prepares for. The name itself preserves that relationship, hallow being an old word for saint, and the eve simply the night before. Its textual basis is liturgical rather than scriptural, a vigil observance of the kind the Western church attached to many major feasts, kept with fasting and prayer in the medieval calendar before the feast proper began at sunset the following evening under the older reckoning of the liturgical day.

The date's popular association with the dead, costumes and bonfires has often been read as a straight survival of the Gaelic festival of Samhain, which fell on the same day, but the historical case for that continuity is weaker than the popular account assumes. What the record supports is a slower convergence, a Christian vigil and a preexisting seasonal turning point sharing a date and gradually exchanging customs across centuries in the British Isles and Ireland, rather than one continuous unbroken practice handed down from pre Christian religion. The layered result, souling, bonfires and later costumed guising, reflects that long convergence rather than a single origin, and the feast it keeps vigil for remains the fixed point the date was set to serve.