John Lightfoot on Luke 2:21

[And when eight days were accomplished for the circumcision of the child.] "The disciples of R. Simeon Ben Jochai asked him, Why the law ordained circumcision on the eighth day? To wit, lest while all others were rejoicing, the parents of the infant should be sad. The circumcision therefore is deferred till the woman in childbed hath got over her uncleanness." For, as it is expressed a little before, "The woman that brings forth a man-child is prohibited her husband the space of seven days, but on the seventh day, at the coming in of the evening which begins the eighth day, she washeth herself, and is allowed to go in unto her husband." If she came nigh him within the seven days she made him unclean. On the eighth day, therefore, Joseph addresseth himself to make provision for his wife, and to take care about the circumcision of the child.