That’s the actual birth.
Annunciation. Several Christian churches have a holiday dedicated to it, the date for which has, as has been pointed out, been selected as the day nine months before Christmas. It seems, however, that Conception of Christ (Latin conceptio Christi) is an acceptable alternative term for it.
The Annunciation is the announcement that Mary “would conceive”, not the actual conception (although they were presumably concurrent events).
I prefer to envision the Angel Gabriel here as Paul Anka:
Having God’s baby
What a wonderful way to show how much you love Him…
Well, we do have the Feast of the Epiphany on January 6, the traditional date of Jesus’ circumcision.
And from that, we have the Holy Foreskin, which a dozen or so churches have/had as a relic.
Actually that’s New Year’s day, not the 6th. A week after the birth.
Jan 1 got redenominated as primarily Feast of Mary Mother of God after Vat II - I suppose the church may have been tired of having to explain “circuncision” to the children at catechism. But it was and is still a celebration of Jesus’ simultaneous human and divine natures.
The Epiphany on January 6 commemorates the visit of the Magi.
As has been pointed out, the Feast of the Circumcision (now the Solemnity of Mary), is celebrated on January 1.
The correct answer to the question in the OP, the Incarnation, was given in the second post.
From the Wiki article on the Incarnation:
You can’t expect medical precision from theology. Christ was incarnated (made flesh) when he was conceived, which would have taken place at or shortly after the Annunciation. But the most obvious evidence of his incarnation was his birth at Christmas. Hence that’s when his incarnation is mainly commemorated.
As already pointed out, the dates when Christ was born and when he was conceived do not conform to when they actually would have taken place.
Is there a standard term for Jesus’ conception?
The Ejaculate Deception?
Cf. the standard reference here, The SDGQ Sacred Foreskin Hijack Thread.
Says who? Maybe Mary had a Jesus-shaped tumor growing inside her for nine months, and upon the Incarnation the tumor transmuted into Jesus. Coulda been two seconds prior to birth.