VI. Priests and deacons may not be ordained on weekdays any more than bishops.
You shall take order that this letter reach the knowledge of all the brethren, so that no one hereafter find an opportunity to excuse himself through ignorance in observing these things which we command. We have directed our letter of admonition to the metropolitans themselves also of the several provinces, that they may know that they must obey the Apostolic injunctions, and that they obey us in beginning to obey you, brother, our delegate according to what we have written. We hear, indeed, and we cannot pass it over in silence, that only bishops are ordained by certain brethren on Sundays only; but presbyters and deacons, whose consecration should be equally solemn, receive the dignity of the priestly office indiscriminately on any day, which is a reprehensible practice contrary to the canons and tradition of the Fathers, since the custom ought by all means to be kept by those who have received it with respect to all the sacred orders: so that after a proper lapse of time he who is to be ordained a priest or deacon may be advanced through all the ranks of the clerical office, and thus a man may have time to learn that of which he himself also is one day to be a teacher. Dated the 12th of January, in the consulship of Theodosius (18th time) and Albinus (444).
Source: Letters (New Advent)