Objection 1. It would seem that Christ was not born at a fitting time. Because Christ came in order to restore liberty to His own. But He was born at a time of subjection--namely, when the whole world, as it were, tributary to Augustus, was being enrolled, at his command as Luke relates (2:1). Therefore it seems that Christ was not born at a fitting time.
Objection 2. Further, the promises concerning the coming of Christ were not made to the Gentiles; according to Romans 9:4: "To whom belong... the promises." But Christ was born during the reign of a foreigner, as appears from Matthew 2:1: "When Jesus was born in the days of King Herod." Therefore it seems that He was not born at a fitting time.
Objection 3. Further, the time of Christ's presence on earth is compared to the day, because He is the "Light of the world"; wherefore He says Himself (John 9:4): "I must work the works of Him that sent Me, whilst it is day." But in summer the days are longer than in winter. Therefore, since He was born in the depth of winter, eight days before the Kalends of January, it seems that He was not born at a fitting time.
On the contrary, It is written (Galatians 4:4): "When the fulness of the time was come, God sent His Son, made of a woman, made under the law."
I answer that, There is this difference between Christ and other men, that, whereas they are born subject to the restrictions of time, Christ, as Lord and Maker of all time, chose a time in which to be born, just as He chose a mother and a birthplace. And since "what is of God is well ordered" and becomingly arranged, it follows that Christ was born at a most fitting time.
Reply to Objection 1. Christ came in order to bring us back from a state of bondage to a state of liberty. And therefore, as He took our mortal nature in order to restore us to life, so, as Bede says (Super Luc. ii, 4,5), "He deigned to take flesh at such a time that, shortly after His birth, He would be enrolled in Caesar's census, and thus submit Himself to bondage for the sake of our liberty."
Moreover, at that time, when the whole world lived under one ruler, peace abounded on the earth. Therefore it was a fitting time for the birth of Christ, for "He is our peace, who hath made both one," as it is written (Ephesians 2:14). Wherefore Jerome says on Isaiah 2:4: "If we search the page of ancient history, we shall find that throughout the whole world there was discord until the twenty-eighth year of Augustus Caesar: but when our Lord was born, all war ceased"; according to Isaiah 2:4: "Nation shall not lift up sword against nation."
Again, it was fitting that Christ should be born while the world was governed by one ruler, because "He came to gather His own [Vulgate: 'the children of God'] together in one" (John 11:52), that there might be "one fold and one shepherd" (John 10:16).
Reply to Objection 2. Christ wished to be born during the reign of a foreigner, that the prophecy of Jacob might be fulfilled (Genesis 49:10): "The sceptre shall not be taken away from Juda, nor a ruler from his thigh, till He come that is to be sent." Because, as Chrysostom says (Hom. ii in Matth. [Opus Imperf., falsely ascribed to Chrysostom), as long as the Jewish "people was governed by Jewish kings, however wicked, prophets were sent for their healing. But now that the Law of God is under the power of a wicked king, Christ is born; because a grave and hopeless disease demanded a more skilful physician."
Reply to Objection 3. As says the author of the book De Qq. Nov. et Vet. Test., "Christ wished to be born, when the light of day begins to increase in length," so as to show that He came in order that man might come nearer to the Divine Light, according to Luke 1:79: "To enlighten them that sit in darkness and in the shadow of death."
In like manner He chose to be born in the rough winter season, that He might begin from then to suffer in body for us.
Source: Summa Theologica (New Advent)