The Saturdays of commemorations of the dead are called ancestral Saturdays (the first universal commemoration on Meat Fare Saturday, the second, third, and fourth Saturdays of Great Lent, Trinity Saturday, and St. Demetrius Saturday). Why do these take place specifically on Saturdays? What are the historical roots of this tradition? They were not all instituted at the same time.
God blessed the seventh day and sanctified it, because in it he ceased from all His works which God began to do (Gen. 2:3). Saturday (Sabbath) for the Jews was a day of festive rest. Christ’s resurrection placed the beginning of the new Israel: a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light (1 Pet. 2:9). The resurrection day of the Savior of the World became the seventh, festive day that completes the week. Sunday [in Russian, voskresenie, meaning “resurrection”) is a day of prayer in church at Divine Liturgy and pious rest. From a day of earthly rest, Saturday became a symbol of joyous rest in the Kingdom of Heaven: There remaineth therefore a rest to the people of God. For he that is entered into his rest, he also hath ceased from his own works, as God did from his (Heb. 4:9–10). This is where the custom, fixed by the Church typicon, came from of having special services on Saturday for the commemoration of the dead.
The establishment of the universal ancestral Meat Fare Saturday dates back to the first century of Christianity. In the Synaxarion for this day (The Lenten Triodion) it says that the holy fathers established, having received it from the holy Apostles, that on this day should be commemorated all people from the ages who have reposed in faith and piety. This day was chosen because Meat Fare week reminds us of the future Last Judgment. On the eve of this day, Saturday, as if preceding the Last Judgment, the Church prays especially for all of its reposed children, begging the Lord to have mercy on them and make them partakers of blessed eternal life.
Just like Meat Fare Saturday, ancestral Saturday before Holy Trinity Day (Pentecost) is called universal. The custom of commemorating the dead on that day also dates from Apostolic times. On that Saturday, prayers are raised for all people from the ages who have died with the hope of resurrection and eternal life, because on the day of Pentecost, the Kingdom of Christ appeared by the descent of the Life-creating Most Holy Spirit. Therefore, on the eve of this feast, the Church prays for all of its children who have reposed up to then, that they would be gathered into the Heavenly Kingdom and vouchsafed eternal blessed life.
During Great Lent, the Church does not serve the usual daily commemorations of the dead (pannikhidas, litias). As a supplement to this, so that the dead are not deprived of the Church’s saving intercessions, the commemorations were established for the Saturdays of the second, third, and fourth weeks of Lent.
St. Demetrius Saturday is celebrated on the nearest Saturday before the commemoration day of Great Martyr Demetrius of Thessalonka (October 26/November 8). This Saturday commemoration was established in the Russian Church after the battle of Kulikovo (September 8, 1380), with the blessing of St. Sergius of Radonezh. At first, commemorations were served for all soldiers who fell in that battle. Eventually, St. Demetrius Saturday came to be a day when all reposed Orthodox Christians are commemorated.