Historians believe the Church chose this date for strategic and symbolic reasons rather than chronological accuracy:
The phrase (The First Christmas) carries two distinct meanings. It can refer to the biblical and historical origins of the Nativity in Bethlehem, or it can refer to the historical development of the holiday itself—when and why Christians first began celebrating the birth of Jesus on December 25th. Il primo Natale
Here is an informative overview covering both interpretations. 1. The Biblical Narrative: The Nativity Historians believe the Church chose this date for
St. Francis of Assisi created the first presepe (nativity scene) in 1223 in Greccio, Italy, to make the story more accessible to the public. Il primo Natale