From a Pantheon Rededicated to a Feast for Every Church

All Saints falls on the first of November in the Western church, the date this rule records, but the feast did not begin there. Pope Boniface the Fourth rededicated the Pantheon in Rome in the year six hundred nine as a church to Mary and all the martyrs, and for over a century the West kept a commemoration of all the saints in May rather than November. The move to the first of November is credited to Pope Gregory the Third in the eighth century, who dedicated an oratory in Saint Peter's to all the saints on that date, and the observance became obligatory across the Frankish kingdom under Pope Gregory the Fourth around eight hundred thirty five, from which point the November date spread through the rest of the Western church.

The Eastern churches kept, and largely still keep, an older logic instead, honoring all saints together on the first Sunday after Pentecost rather than on a fixed autumn date, tying the commemoration to the outpouring of the Holy Spirit rather than to the turn of the liturgical year toward winter. The two calendars therefore agree on the idea, a single day honoring every saint the church cannot name individually, while placing it in entirely different seasons and for different theological reasons, one an autumn feast answering to Rome's own liturgical history and one a Pentecost season feast answering to a different set of associations entirely.