Objection 1. It would seem that weddings ought not to be forbidden at certain times. For marriage is a sacrament: and the celebration of the others sacraments is not forbidden at those times. Therefore neither should the celebration of marriage be forbidden then.
Objection 2. Further, asking for the marriage debt is more unbecoming on feast days than the celebration of marriage. Yet the debt may be asked for on those days. Therefore also marriages may be solemnized.
Objection 3. Further, marriages that are contracted in despite of the law of the Church ought to be dissolved. Yet marriages are not dissolved if they be contracted at those times. Therefore it should not be forbidden by a commandment of the Church.
On the contrary, It is written (Ecclesiastes 3:5): "A time to embrace, and a time to be far from embraces."
I answer that, When the newly married spouse is given to her husband, the minds of husband and wife are taken up with carnal preoccupations by reason of the very newness of things, wherefore weddings are wont to be signalized by much unrestrained rejoicing. On this account it is forbidden to celebrate marriages at those times when men ought especially to arise to spiritual things. Those times are from Advent until the Epiphany because of the Communion which, according to the ancient Canons, is wont to be made at Christmas (as was observed in its proper place, III, 30), from Septuagesima until the octave day of Easter, on account of the Easter Communion, and from the three days before the Ascension until the octave day of Pentecost, on account of the preparation for Communion to be received at that time.
Reply to Objection 1. The celebration of marriage has a certain worldly and carnal rejoicing connected with it, which does not apply to the other sacraments. Hence the comparison fails.
Reply to Objection 2. There is not such a distraction of minds caused by the payment of a request for the debt as by the celebration of a marriage; and consequently the comparison fails.
Reply to Objection 3. Since time is not essential to a marriage contracted within the forbidden seasons, the marriage is nevertheless a true sacrament. Nor is the marriage dissolved absolutely, but for a time, that they may do penance for having disobeyed the commandment of the Church. It is thus that we are to understand the statement of the Master (Sent. iv, D, 33), namely that should a marriage have been contracted or a wedding celebrated at the aforesaid times, those who have done so "ought to be separated." Nor does he say this on his own authority, but in reference to some canonical ordinance, such as that of the Council of Lerida, which decision is quoted by the Decretals.
Source: Summa Theologica (New Advent)