It started when some families started celebrating Early Christmas. It was a matter of convenience. The modern age created more complicated families, and those families would celebrate early, at whatever time fit all their complicated schedules. The most common Early Christmas was the weekend before the 25th. As the custom spread, retailers and advertisers pushed their Christmas blitzes earlier and earlier. More locally, communities pushed the date earlier and earlier in a form of competition. Children, excited and gloating over the bounties of their Early Christmasses, infected families with envy. Children who hadn't yet celebrated the commercial holiday cried to their parents, who planned an earlier Christmas for the next year.
Eventually, communities stabilized by mutual cease-fire. Many wound up holding Early Christmas in early November, but some places celebrated as early as July. Authorized Christmas dates appeared in neighborhood homeowner association agreements.
Of course, some communities declared that only certain dates were acceptable. December 25th was a popular choice in the Bible Belt. But the upheaval opened the door for some to declare that late September was Jesus's true birthday. Then believers began to insist that others celebrate on the "correct" date.
Thus began the First Christmas War.