The Church Fathers included the commemoration of many Martyrs and other Saints in the liturgical year. The II Vatican Council reminds us that the Martyrs and Saints, being “raised to holiness by abundant graces of God and already in possession of their eternal salvation, sing constant praises to God in heaven and offer prayers for us” (ct. Decree on the Liturgy, n. 104). By celebrating the passage of the Saints from the earth to heaven, the Church also proposes them to us as so many examples of genuine Christian living.
The veneration of the Saints has a similar purpose. This began in the first century, first the Veneration of the Martyrs and then of the Apostles. Soon other Saints were added. Between the fourth and the fifth centuries the veneration of the Saints became a general practice, ceding the first place to St. John the Baptist (after the Blessed Mother and the Angels), in view of Christ’s testimony: “There is no one greater than John!” (Lk. 7:28). The Saints usually are commemorated on the anniversary of their death, since the departure of those “that died in the Lord” (Rom. 14:8) was considered by the Christians as a day of birth to a new and happy life with God. (Byzantine Leaflect Series "THE LITURGICAL YEAR according to THE BYZANTINE TRADITION"