The circumcision of Jesus is recorded in Gospel of Luke chapter 2, thus: “And when eight days were fulfilled to circumcise the child, his name was called Jesus, the name called by the angel before he was conceived in the womb.”
The eighth day after his birth is traditionally observed on 1st January. This is in keeping with the Jewish law which holds that males should be circumcised eight days after birth during a Brit milah ceremony, at which they are also given their name.
The Feast of the Circumcision is first recorded from a church council held at Tours in 567, although it was clearly already long-established by then. It has traditionally been seen, as explained in the popular 14th-century work the Golden Legend, as the first time the blood of Christ was shed, and thus the beginning of the process of the redemption of man, and a demonstration that Christ was fully human, and of his obedience to Biblical law.
Picture above: The Circumcision of Christ by Menologion of Basil II (11th century), Anonymous. This work is in the public domain in its country of origin and other countries and areas where the copyright term is the author's life plus 100 years or fewer.