Answer:
It comes from the same place that any claim about gods comes: the human imagination.
Step-by-step explanation:
Various scholars have come up with different times of year for the guy to have been born - if indeed he existed as a single man, and not a pastiche of several itinerant preachers of the time. The one that has the least support in any literature that survives, is December 25.
The reason that date was picked was because the early church wanted to convert pagans, and a common pagan festival was held at the Winter solstice, when the days finally stopped getting shorter and began getting longer again (yay! drinking, merriment, and fellowship, and a new year coming!). So they declared a holiday/birthday at that time, so the pagans would not lose their celebration if they converted. At the time, the officers of the early church had no idea that the southern hemisphere was seeing exactly the opposite trend-reversal, or they didn’t care, because the pagans they were interested in recruiting were local (mostly within and bordering the Roman empire). They didn’t know or care about south Africans or Australian aborigines or South American indigenes who would have been looking to observe their SUMMER solstice and the less-happy trend toward darkness and shortened days.