In the faith's early days Christians solemnized the anniversary of a martyr's death at the place of his or her martyrdom. Frequently groups of martyrs suffered on the same day, which naturally led to a joint commemoration. By the fourth century, neighboring dioceses began to transfer relics and join in a common feast. In the persecution of Diocletian, the number of martyrs became so great that a separate day could not be assigned to each so the Church, determining that every martyr should be venerated, appointed a common day for all.
This practice originally began in Antioch. At first only martyrs and St. John the Baptist were honored by a special day. Other saints were gradually added and increased in number when a regular process of canonization was established.
Almighty God, you have knit together your elect in one communion and fellowship in the mystical body of your Son Christ our Lord: Give us grace so to follow your blessed saints in all virtuous and godly living, that we may come to those ineffable joys that you have prepared for those who truly love you; through Jesus Christ our Lord, who with you and the Holy Spirit lives and reigns, one God, in glory everlasting. Amen.