Leviticus 12:4
And she shall then continue in the blood of her purifying three and thirty days; she shall touch no hallowed thing, nor come into the sanctuary, until the days of her purifying be fulfilled. And she shall then continue in the blood of her purifying three and thirty days; she shall touch no hallowed thing, nor come into the sanctuary, until the days of her purifying be fulfilled.
After the birth of a son the mother was to remain for thirty-three days in the blood of her purifying, touching no hallowed thing and not coming into the sanctuary until the days of her purifying were fulfilled. Mary kept this requirement after the birth of Jesus as part of the appointed rites of purification and presentation. With the preceding days of separation added to these thirty-three days, the law accounts for Jesus being brought to the temple when He was about six weeks old. That careful observance was the course of Christ as humanity's substitute, conforming to the law in every particular.