Deuteronomy 21:12-13

She shall shave her head This was one of the external signs of mourning, Leviticus 19:27; Leviticus 21:5. Shall pare her nails This also seems to have been done in mourning. In the original it is, Shall make her nails, which may be understood of letting her nails grow, which to us seems more suitable to a state of mourning. But this is to be resolved entirely into the fashion of countries. Poole thinks that both of these things were rather to be done in token of her renouncing her heathenish idolatry and superstition, and of her becoming a new woman, and embracing the true religion. She shall put the raiment of her captivity off from her

That is, as the French renders the words more clearly, the raiment which she wore when she was taken captive. Instead of the fine clothes wherein she had been taken captive, she was to put on sordid apparel, which was the habit of mourners. And shall bewail her father and her mother Either their death, or, which was in effect the same, her final separation from them, being now to forget all her former relations.