I believe the nativity began long before the donkey and the manger; long before Caesar's infamous decree and long before Mary was even born. I like to think it began eons before when prophets of old began prophesying of his coming.
The first prophet that comes to my mind is Isaiah. He prophesied of Christ's birth in Isaiah 7:14 more than 500 years before His birth. It says, "Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel."
Isaiah also declared, "For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace" (Isaiah 9:6), and that He would rule over all the earth (Isaiah 9:7).