Matthew Poole and continuators on Luke 2:21

21 And when eight days were accomplished for the circumcising of the child, his name was called JESUS, which was so named of the angel, before he was conceived in the womb.

The time Prescribed by the Divine Law for Circumcision, was the Eighth Day, Gen. 17.12. Levit. 12.3. he was indeed the Law-giver, and as such not tyed to the observance of the Law. But he was also made of a Woman, made under the Law, Gal. 4.4. and the Law was, Lev. 12.2. That if a Woman had conceived Seed, and Born a Man-Child in the Eighth Day, the Flesh of his fore-skin should be Circumcised. He was to make himself appear the Son of Abraham, and so this was Gods Covenant, Gen. 17.10. with Abraham and his Seed after him, every Man-Child among you shall be Circumcised. This Law Christ was bound to fulfil, and by the fulfilling of it in this point, he shewed himself a Debtor to the whole Law, Galath. 5.3. and by his observance of it, he was to teach us our Duty. He was to be a Minister of the Circumcision, Rom. 15.8. and to the Circumcision which they would never have allowed him to be, had not he himself been Circumcised, upon which account Paul took Timothy, and Circumcised him, Acts 16.3. By his Circumcision also we were to be Circumcised, with the Circumcision made without Hands, in putting off the Body of the Sin of the Flesh, Coloss. 2.11. It was therefore reasonable and necessary that Christ should be Circumcised the eighth Day. His Name was called Iesus, it was in Circumcision before Witness Publickly declared to be so, for God by his Angel had given him his Name, Matth. 1.21. we read of four under the Old Testament, to whom God gave Names, before they were Born, Isaac, Gen. 17.19. Iosiah, 1 Kings 13.2. Ishmael, Gen. 16.11. Cyrus, Isaiah 44.28. and in the New Testament to Iohn the Baptist, and to Iesus Christ. Which lets us know the certainty to God of future Contingencies, for tho the Parents of Ishmael, and Isaac, Iohn Baptist, and Christ, imposed those Names in obedience to the Command of God, and there was but a small time betwixt, the giving of these four their Names, and their Birth, yet the Case was otherwise as to Iosiah and Cyrus.

Marginal references: Chap. 1.31.