According to the Catholic Church, Jesus may not have actually born on December 25 nor is this proclaimed. We celebrate it on this day. It's a celebration of the Incarnation, not a memorial of a specific day. One reason December 25 may have been thought fitting is its proximity to the winter solstice. After that date the days start to become longer, and thus it is at the beginning of a season of light entering the world (cf. John 1:5).
According to the Catholic Church, Jesus may not have actually born on December 25 nor is this proclaimed. We celebrate it on this day. It's a celebration of the Incarnation, not a memorial of a specific day. One reason December 25 may have been thought fitting is its proximity to the winter solstice. After that date the days start to become longer, and thus it is at the beginning of a season of light entering the world (cf. John 1:5).