Perhaps we will never be able to understand fully what miracle took place two thousand years ago. We can only name some sides of this incomprehensible event. In the Nativity we are given a stunning revelation of God about Himself: it turns out that the One God has an Only-begotten Son, and this does not contradict the unity of God. But beyond this, the Son, being "greater" than the whole world, enters and is contained within this world, coming into it as a human Infant. He is born into this world like all children, passing through every stage of development in the womb, and this is the One in whom all the fullness of Deity dwells bodily. He receives His human body from the Virgin, who preserved her virginity even in the Nativity.
Further, it turns out that not all time is alike, since there is a "fullness of time": the moment when God's centuries-long care for the upbringing of the human person leads to the appearance, in this world lying in evil, of such a young Virgin, one who was able to receive into herself the fullness of God's grace... One can keep listing more and more... But it is better simply to rejoice in this miracle and bring this joy to all people, who need joy so much.