For two centuries after Christ was born, no one cared when He was born. Death days were more important. Early in the 300s AD, the Church decided to celebrate His birth on December 25, to combat festivities on that date by a rival religion, Mithraism, that threatened Christianity.
Mithra, the sun god, was so popular with the people that Emperor Aurelian proclaimed it the official state religion in the Roman Empire. Romans loved extended parties, and this celebration went on for days.
Several dates were discussed, including March, because the Bible says shepherds were watching their sheep by night and they only did this in spring, at lambing time. The December date was chosen so the Christians had their own way to celebrate their religion.
Christmas was made permanent in 337 in the Western world, when the Roman Emperor Constantine was baptized, uniting the Church and the Roman Empire. Christianity became the official state religion.