Originally, early Christians held a Mass service for Christ (hence Christmas’ name) but also debated the actual birth date of Jesus, which was never stated in the Bible.
Indeed, even the year was, and still is, debated. Many say that Jesus was born in another season, maybe in the month of September.
Another theory is that Pope Julius saw the date coincided with the winter solstice and the Jewish Hannukah, which were already times of celebration, so let the festivities be enhanced during this period.
Besides, cattle had been slaughtered and wine and beer had fermented during the winter so everyone was ready to consume fresh produce in abundance.
The first recorded celebration of Christmas took place in 336 AD under the reign of the Roman Emperor Constantine, who had slowly converted to Christianity, with Pope Julius making it official around 350.
In 529, Christmas Day was declared an official holiday.
Another suggestion is that March 25 was not only the date that Mary was told she would be having a sacred baby in nine months but also the date that Jesus died when he was an adult.