It was argued, among Bible scholars, whether Jesus was born on or around December 25th because, among other reasons, in the Holy Land it would have been too cold in December for shepherds to have their flocks outdoors at night. The Bible, and the early Christians, did not designate a date nor even a month for the Nativity.
Apparently the date of December 25th was settled on primarily because it was longest night (or, considering the primitive time keeping equipment then existing, was mistakenly thought to be the longest night) and it seemed that all the Gospel events that apparently took place at night would have needed as much time as possible to unfold.
Whether Dec 25th was chosen because it was already being used by various pagans is debatable, but it is certain that a lot of pagan cultures had already noticed that the longest night was on or around Dec 25th and were marking that night in various ways.
Another possible reason for picking Dec 25th, or while that date might have prevailed over other candidate dates, is that it comes in the midst of winter. In a pre-industrial age the vast majority of the population in Europe would have dug themselves in as the snows began in November or October, resigned to see no one outside the household and eat nothing but stored roots until the spring thaw; celebrating Christmas on Dec 25th gave them an opportunity and excuse to dig themselves out, socialize and have some amusement midway through this hibernation.
Re: “Apparently the date of December 25th was settled on primarily because it was longest night”
"The Deus Sol Invictus "God, the Unconquerable Sun" cult has an eastern, Syrian origin; and it was centered on the winter solstice. It was first imported to Rome in 219 A.D. By 274 A.D., it was proclaimed the dominant religion of the Roman state by the emperor Aurelian. It was during the reign of Constantine the Great (306-337 A.D.) that the cult of Deus Sol Invictus reached extraordinary heights, so that Constantine's reign was even spoken of as the Sun Emperorship. The heart of the cult centered on the celebration of the Dies Natali Invicti on December 25, and it was this date on which Constantine subsequently based the date of Christmas. Constantine established the date of Christ's birth as falling on December 25 - thus simultaneously incorporating the Saturnalia, "the merriest festival of the year" (Cattalus), the official Julian calendar date for the solstice, and the symbolism of Mithra's birth [from the cosmic egg on the winter solstice] into Christianity."
In 567 A.D. the Council of Tours declared the Twelve Days of Christmas from Christmas to Epiphany (December 25th to January 6th) festival time and made the birth and baptism of Jesus Christ a single celebration. Christmas Day was not an official Roman holiday until Charlemagne became emperor in 800 A.D.