Christians: why do you celebrate the birth of Jesus....?

Yeah. That's not his actual birthday. But its the day that falls with Christ' birth in the Liturgical year. The Catholic Church was creating the Liturgical Year and Christs Birthday fell somewhere in December. They couldn't pick a day, so, to try to make a conversion from pagansim to Christianity they put it on the 25th, the same day as the Birth of another Pagan God.