We don’t celebrate birth which is pagan, but we celebrate Jesus death which also has pagan roots (spring equinox) and the story of Jesus, including his virgin birth (Romulus and Remus, the autumn equinox), 40 days in the wilderness (weeping for Tammuz) and 3 years ministry and apocalyptic message, turning water into wine (Dionysus, god of festivities) and death by crucifixion and resurrection (Zeus) all based on pagan stories.
But birthday CAKE, a thing that wasn’t invented for 1400 years after Jesus’ death, and birthday parties, a thing that started around the time CT Russell’s parents were born, that is pagan.