The definite article. A three-word phrase indicating specificity, that sets something apart from all others.
I am Bo Wagner, but I am not “the Bo Wagner” since a quick Google search reveals singers, medical doctors, insurance salesmen and lawyers of the same name.
But when it comes to the birth of Jesus, it really is appropriate to call it “The Birthday.” Yes, others (many, many others) have been born and thus had a birthday, but no one anywhere at any time has had a birthday quite like his.
It started in the garden of Eden, really. When God spoke to the serpent in Genesis 3:15, he informed him that he would send the “seed of the woman” to bruise the serpent’s head. That is the Protoevangelium, the first Gospel, the promise of the messiah. In practical terms, though, it was a birth announcement 4,000 years before the fact. “The” indeed.
Fast forward 3,300 years or so to the time of Isaiah. Two enemy kingdoms were coming against Judah. Ahaz, the king of Judah, was rightly terrified. But God sent Isaiah the prophet to meet the king and to deliver a message to him. It was a message of comfort for the present and for the near future but, in the midst of that, God also sent a prophecy of the distant future. He said in Isaiah 7:14: “Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.”