H/T: Fr. Ted's Blog
The first indication of this universal commemoration of the dead on Saturday of Meatfare week appears in the Typikon of the Great Church (9th-10th century). It is possible that it was instituted in connection with the commemoration on Meatfare Sunday of the last judgement. The Typicon St. Alexios the Studite (11th century) describes an ordo of the office very similar to what we celebrate today.
Bishop Afanasii (Sakharov) believed that, during these two days, the Church prayed in a more intense way for the repose of all the dead, familiar and stranger, known and unknown, of every age and circumstance, of all times and all peoples, of all who have died since the beginning of the world. According to him, this is the reason the Church put aside the commemoration of saints from the Menaion, in order to dedicate itself fully to prayer for the dead. Indeed, in contrast with other Saturdays, when the commemoration of the of the dead follows the glorification of all the saints, here the memorial of the dead takes up the entire focus of the liturgical celebration…
These rubrics reflect an ancient practice, attested to by Canon 51 of the Council of Laodicea [363-364AD], which instructs that the memory of the martyrs should not be celebrated during the forty days of Lent, but on Saturdays and Sundays. Theodore Balsamon (c. 1140-c. 1195), the great Byzantine canonist, already considered that this canon concerning the commemoration of martyrs applied equally to the commemoration of the dead.
- Archimandrite Job Getcha
The Typikon Decoded, pp 147-148, 186