II. Fixed days should be observed for ordaining priests and deacons.
That therefore which we know to have been very carefully observed by our fathers, we wish kept by you also, viz. that the ordination of priests or deacons should not be performed at random on any day: but after Saturday, the commencement of that night which precedes the dawn of the first day of the week should be chosen on which the sacred benediction should be bestowed on those who are to be consecrated, ordainer and ordained alike fasting. This observance will not be violated, if actually on the morning of the Lord's day it be celebrated without breaking the Saturday fast: for the beginning of the preceding night forms part of that period, and undoubtedly belongs to the day of resurrection as is clearly laid down with regard to the feast of Easter. For besides the weight of custom which we know rests upon the Apostles' teaching, Holy Writ also makes this clear, because when the Apostles sent Paul and Barnabas at the bidding of the Holy Ghost to preach the gospel to the nations, they laid hands on them fasting and praying: that we may know with what devoutness both giver and receiver must be on their guard lest so blessed a sacrament should seem to be carelessly performed. And therefore you will piously and laudably follow Apostolic precedents if you yourself also maintain this form of ordaining priests throughout the churches over which the Lord has called you to preside: viz. that those who are to be consecrated should never receive the blessing except on the day of the Lord's resurrection, which is commonly held to begin on the evening of Saturday, and which has been so often hallowed in the mysterious dispensations of God that all the more notable institutions of the Lord were accomplished on that high day. On it the world took its beginning. On it through the resurrection of Christ death received its destruction, and life its commencement. On it the apostles take from the Lord's hands the trumpet of the gospel which is to be preached to all nations, and receive the sacrament of regeneration which they are to bear to the whole world. On it, as blessed John the Evangelist bears witness when all the disciples were gathered together in one place, and when, the doors being shut, the Lord entered to them, He breathed on them and said: “Receive the Holy Ghost: whose sins you have remitted they are remitted to them: and whose you have retained, they shall be retained.” On it lastly the Holy Spirit that had been promised to the Apostles by the Lord came: and so we know it to have been suggested and handed down by a kind of heavenly rule, that on that day we ought to celebrate the mysteries of the blessing of priests on which all these gracious gifts were conferred.
Source: Letters (New Advent)