Through the Apostolic Constitutions (Book VIII, ch. 42), the Church of Christ has received the custom to make commemorations for the departed on the third, ninth, and fortieth days after their repose. Since many throughout the ages, because of an untimely death in a faraway place, or other adverse circumstances, have died without being deemed worthy of the appointed memorial services, the divine Fathers, being so moved in their love for man, have decreed that a common memorial be made this day for all pious Orthodox Christians who have reposed from all ages past, so that those who did not have particular memorial services may be included in this common one for all. Also, the Church of Christ teaches us that alms should be given to the poor by the departed one's kinsmen as a memorial for them.
Besides this, since we make commemoration tomorrow of the Second Coming of Christ, and since the reposed have neither been judged, nor have received their complete recompense (Acts 17:31; II Peter 2:9; Heb. 11:39-40), the Church rightly commemorates the souls today, and trusting in the boundless mercy of God, she prays Him to have mercy on sinners. Furthermore, since the commemoration is for all the reposed together, it reminds each of us of his own death, and arouses us to repentance.
Apolytikion in the Plagal of the Fourth Tone
Only Creator who out of the depths of wisdom lovingly govern all things and upon all bestow what is accordingly best for them, give rest to the souls of Your servants, for they have placed their hope in You, our Author and Maker and God.
Kontakion in the Plagal of the Fourth Tone
Give rest, O Christ, among the Saints to the souls of Your servants, where there is no pain, no sorrow, no grieving, but life everlasting.
- Reading from Greek Orthodox Church of America website