The Old Testament took note of this number eight, called by us in Latin an octave, for Ecclesiastes says, “Give a portion to those seven, and also to those eight.” The seven of the Old Testament is the eight of the New, since Christ arose and the day of the new salvation has shed light upon all. It is the day of which the prophet says, “This is the day which the Lord has made; let us be glad and rejoice in it.” Letter , To Horontianus.
Before the Lord’s resurrection there was rest for the departed but resurrection for none: “Rising from the dead he dies no more, death has no more dominion over him.” But after such resurrection had taken place in the Lord’s body, so that the head of the church might foreshadow what the body of the church hopes for at the end, then the Lord’s day—that is the eighth, which is also the first—began to be observed.
The one who “gives a part to the seven” commits to the Old Testament, which was before the arrival of the Savior. The number seven indicates the institution of the sabbath. The one who “gives a part to the eight” is the one who believes in the resurrection of the Savior, since he came after the sabbath. The Jews who “give a part to the seven” have not “given a part to the eight” and therefore they were not saved. The heretics … rejected the law and the Old Testament; since they did not “give a part to the seven,” they miss the goal.
They need not be hesitant to pray at their own special hours, that is, the second, fourth, fifth, seventh, eighth, tenth, and eleventh, inasmuch as seven or eight are harmonious with the words of Solomon: “Make seven or eight portions.” [This is] in order that they may be able to climb through the sevenfold grace of the Spirit and the eight beatitudes on the day of resurrection with unhampered tread up the ladder of Jacob by its fifteen steps to the region of heaven, where Christ is resplendent above.
Eight. To as many as thou art able, (Calmet) especially to those who are of the household of faith, (Galatians vi. 10.; Haydock) whether under the old or the new Testament, signified by the numbers, seven and eight. (Worthington) (St. Jerome)
Mandatum accipis octo illis partem dare, fortasse benediction bus, (St. Ambrose in Luke vi. n. 49.) which intimates, that we must apply ourselves to the pursuit of all virtues, as the number eight denotes perfection. (Calmet)