We may make no promise that is wrong, and if we have made an unjust oath, we may not keep it. It is shown that Herod sinned in this respect. The vow taken by Jephtha is condemned, and so are all others which God does not desire to have paid to Him. Lastly, the daughter of Jephtha is compared with the two Pythagoreans and is placed before them.
76. A man's disposition ought to be undefiled and sound, so that he may utter words without dissimulation and possess his vessel in sanctification; that he may not delude his brother with false words nor promise anything dishonourable. If he has made such a promise it is far better for him not to fulfil it, rather than to fulfil what is shameful.
77. Often people bind themselves by a solemn oath, and, though they come to know that they ought not to have made the promise, fulfil it in consideration of their oath. This is what Herod did, as we mentioned before. For he made a shameful promise of reward to a dancer— and cruelly performed it. It was shameful, for a kingdom was promised for a dance; and it was cruel, for the death of a prophet is sacrificed for the sake of an oath. How much better perjury would have been than the keeping of such an oath, if indeed that could be called perjury which a drunkard had sworn to in his wine-cups, or an effeminate profligate had promised while the dance was going on. The prophet's head was brought in on a dish, and this was considered an act of good faith when it really was an act of madness!
78. Never shall I be led to believe that the leader Jephtha made his vow otherwise than without thought, when he promised to offer to God whatever should meet him at the threshold of his house on his return. For he repented of his vow, as afterwards his daughter came to meet him. He rent his clothes and said: “Alas, my daughter, you have entangled me, you have become a source of trouble unto me.” And though with pious fear and reverence he took upon himself the bitter fulfilment of his cruel task, yet he ordered and left to be observed an annual period of grief and mourning for future times. It was a hard vow, but far more bitter was its fulfilment, while he who carried it out had the greatest cause to mourn. Thus it became a rule and a law in Israel from year to year, as it says: “that the daughters of Israel went to lament the daughter of Jephtha the Gileadite four days in a year.” I cannot blame the man for holding it necessary to fulfil his vow, but yet it was a wretched necessity which could only be solved by the death of his child.
79. It is better to make no vow than to vow what God does not wish to be paid to Him to Whom the promise was made. In the case of Isaac we have an example, for the Lord appointed a ram to be offered up instead of him. Therefore it is not always every promise that is to be fulfilled. Nay, the Lord Himself often alters His determination, as the Scriptures point out. For in the book called Numbers He had declared that He would punish the people with death and destroy them, but afterwards, when besought by Moses, He was reconciled again to them. And again, He said to Moses and Aaron: “Separate yourselves from among this congregation that I may consume them in a moment.” And when they separated from the assembly the earth suddenly clave asunder and opened her mouth and swallowed up Dathan and Abiram.
80. That example of Jephtha's daughter is far more glorious and ancient than that of the two Pythagoreans, which is accounted so notable among the philosophers. One of these, when condemned to death by the tyrant Dionysius, and when the day of his death was fixed, asked for leave to be granted him to go home, so as to provide for his family. But for fear that he might break his faith and not return, he offered a surety for his own death, on condition that if he himself were absent on the appointed day, his surety would be ready to die in his stead. The other did not refuse the conditions of suretyship which were proposed and awaited the day of death with a calm mind. So the one did not withdraw himself and the other returned on the day appointed. This all seemed so wonderful that the tyrant sought their friendship whose destruction he had been anxious for.
81. What, then, in the case of esteemed and learned men is full of marvel, that in the case of a virgin is found to be far more splendid, far more glorious, as she says to her sorrowing father: “Do to me according to that which has proceeded out of your mouth.” But she asked for a delay of two months in order that she might go about with her companions upon the mountains to bewail fitly and dutifully her virginity now given up to death. The weeping of her companions did not move her, their grief prevailed not upon her, nor did their lamentations hold her back. She allowed not the day to pass, nor did the hour escape her notice. She returned to her father as though returning according to her own desire, and of her own will urged him on when he was hesitating, and acted thus of her own free choice, so that what was at first an awful chance became a pious sacrifice.
Source: On the Duties of the Clergy (New Advent)