Objection 1. It would seem that a person ought not to be forbidden to ask for the debt on holy days. For the remedy should be applied when the disease gains strength. Now concupiscence may possibly gain strength on a feast day. Therefore the remedy should be applied then by asking for the debt.
Objection 2. Further, the only reason why the debt should not be demanded on feast days is because they are devoted to prayer. Yet on those days certain hours are appointed for prayer. Therefore one may ask for the debt at some other time.
On the contrary, Just as certain places are holy because they are devoted to holy things, so are certain times holy for the same reason. But it is not lawful to demand the debt in a holy place. Therefore neither is it lawful at a holy time.
I answer that, Although the marriage act is void of sin, nevertheless since it oppresses the reason on account of the carnal pleasure, it renders man unfit for spiritual things. Therefore, on those days when one ought especially to give one's time to spiritual things, it is not lawful to ask for the debt.
Reply to Objection 1. At such a time other means may be employed for the repression of concupiscence; for instance, prayer and many similar things, to which even those who observe perpetual continence have recourse.
Reply to Objection 2. Although one is not bound to pray at all hours, one is bound throughout the day to keep oneself fit for prayer.
Source: Summa Theologica (New Advent)