Albert Barnes Commentary Deuteronomy 21:13

"and she shall put the raiment of her captivity from off her, and shall remain in thy house, and bewail her father and her mother a full month: and after that thou shalt go in unto her, and be her husband, and she shall be thy wife." — Deuteronomy 21:13 (ASV)

Bewail her father and her mother a full month - This is prescribed from motives of humanity, that the woman might have time and leisure to detach her affections from their natural ties, and prepare her mind for new ones.