On circumcision, see Genesis 17:5 note.

And in the eighth day - Before this time the child could scarcely be considered as having strength sufficient to bear the operation; after this time it was not necessary to delay it, as the child was not considered to be in covenant with God, and consequently not under the especial protection of the Divine providence and grace, till this rite had been performed. On circumcision see Clarke's note on Genesis 17:10. Circumcision was to every man a constant, evident sign of the covenant into which he had entered with God, and of the moral obligations under which he was thereby laid. It was also a means of purity, and was especially necessary among a people naturally incontinent, and in a climate not peculiarly favorable to chastity. This is a light in which this subject should ever be viewed, and in which we see the reasonableness, propriety, expediency, and moral tendency of the ceremony.

And in the eighth day the flesh of his foreskin shall be circumcised. Or the foreskin of his flesh, that is, of the man child born according to the law, Genesis 17:12 and this seems to furnish out a reason why a male child was not circumcised before the eighth day, and why it was then, because before that its mother was in her separation and uncleanness, and then was freed from it; and so the Targum of Jonathan. The circumcision of a male child on the eighth day was religiously observed, and even was not omitted on account of the sabbath, when the eighth day happened to be on that; see Gill on John 7:22, John 7:23. It is an observation of Aben Ezra on this place, that the wise men say "in the day", and not in the night, lo, he that is born half an hour before the setting of the sun is circumcised after six days and a half, for the day of the law is not from time to time.

After the expiration of this period, on the eighth day, the boy was to be circumcised (see at Genesis 17). She was then to sit, i.e., remain at home, thirty-three days in the blood of purification, without touching anything holy or coming to the sanctuary (she was not to take any part, therefore, in the sacrificial meals, the Passover, etc.), until the days of her purification were full, i.e., had expired.