Today is the main Christian Feast of the year in Catholic and Protestant traditions (in Orthodoxy it will be celebrated in 14 days, according to the Julian calendar).https://en.wikipedia.org/wiki/Circumcision_of_Jesus
This holiday marks the theoretical beginning of the Christian era and the chronology. Julian and Gregorian calendars begin from the day of Circumcision, instead of from the day of Christmas.
The circumcision of Jesus has traditionally been seen, as explained in the once popular writing "Golden Legend", as the first shedding of Christ’s blood, and thus the beginning of man’s redemption process, and as a demonstration that Christ is fully human and obedient to biblical law.
The ceremony of circumcision in Jewish tradition is also the ceremony of naming, and so significant is the feast in both Christian calendars that it was circumcision, and not the birth of the originally nameless child, that was chosen as the starting point of the Christian era.