Christ was born to this earth about 2000 years ago. This is a one-time event in our history. He walked on this earth, taught his disciples and people, healed many sick people, proclaimed the Gospel and died on the cross for us. All of these events are a part of our history, and will never repeat in the same way again.
But in the church calendar, in our religious practice, we regularly re-experience the birth of Christ, the communion with him and his precious death on the cross. Things repeat in our faith.