Question: Why is Candlemas called the "Purification of our Lady"? From what could she have been purified?
Answer: The Blessed Virgin was born pure, free from every stain of sin, and required absolutely no purification from anything. Nonetheless, she willingly submitted to the ritual of purification required by the Law of Moses of all Jewish women following childbirth. Under the Law, contact with blood made all Jews ritually unclean for a period of time. The blood flow of childbirth required a particularly elaborate period and ritual for purification from this legal uncleanness. Forty days after the birth of a male child or eighty days after the birth of a female:
"When the days of her purification are expired, for a son or for a daughter, she shall bring to the door of the tabernacle of testimony, a lamb of a year old for a holocaust, and a young pigeon or a turtle dove for sin, and shall deliver them to the priest, who shall offer them before the Lord and shall pray for her, and so she shall be cleansed from the issue of her blood" (Leviticus xii: 6-8).
The idea of ritual uncleanness is a sort of fiction of law, generally excluding a person from the full life of the Jewish community, but not bringing with it any moral guilt in the sense that we use that word. Mary's Purification is best understood as an example of her obedience, even to a law that technically did not bind her or her Divine Son.