16 Let us now direct our minds to observe the reason why, in the celebration of Easter, care is taken to appoint the day so that Saturday precedes it: for this is peculiar to the Christian religion. The Jews keep the Passover from the 14th to the 21st of the first month, on whatever day that week begins. But since at the Passover at which the Lord suffered, it was the case that the Jewish Sabbath came in between His death and His resurrection, our fathers have judged it right to add this specialty to their celebration of Easter, both that our feast might be distinguished from the Jewish Passover, and that succeeding generations might retain in their annual commemoration of His Passion that which we must believe to have been done for some good reason, by Him who is before the times, by whom also the times have been made, and who came in the fullness of the times, and who when He said, Mine hour is not yet come, had the power of laying down His life and taking it again, and was therefore waiting for an hour not fixed by blind fate, but suitable to the holy mystery which He had resolved to commend to our observation.
17. That which we here hold in faith and hope, and to which by love we labour to come, is, as I have said above, a certain holy and perpetual rest from the whole burden of every kind of care; and from this life unto that rest we make a transition which our Lord Jesus Christ condescended to exemplify and consecrate in His Passion. This rest, however, is not a slothful inaction, but a certain ineffable tranquillity caused by work in which there is no painful effort. For the repose on which one enters at the end of the toils of this life is of such a nature as consists with lively joy in the active exercises of the better life. Forasmuch, however, as this activity is exercised in praising God without bodily toil or mental anxiety, the transition to that activity is not made through a repose which is to be followed by labour, i.e. a repose which, at the point where activity begins, ceases to be repose: for in these exercises there is no return to toil and care; but that which constitutes rest— namely, exemption from weariness in work and from uncertainty in thought— is always found in them. Now, since through rest we get back to that original life which the soul lost by sin, the emblem of this rest is the seventh day of the week. But that original life itself which is restored to those who return from their wanderings, and receive in token of welcome the robe which they had at first, is represented by the first day of the week, which we call the Lord's day. If, in reading Genesis, you search the record of the seven days, you will find that there was no evening of the seventh day, which signified that the rest of which it was a type was eternal. The life originally bestowed was not eternal, because man sinned; but the final rest, of which the seventh day was an emblem, is eternal, and hence the eighth day also will have eternal blessedness, because that rest, being eternal, is taken up by the eighth day, not destroyed by it; for if it were thus destroyed, it would not be eternal. Accordingly the eighth day, which is the first day of the week, represents to us that original life, not taken away, but made eternal.
Source: Letters (New Advent)