Why do Christians (who worship the exact same Deity) celebrate (Holy)days separately?

I recently went to a Pagan (Holy)day with around 500+ people and talked to around 8 different kinds of pagans celebrating without strife or saying that one way was superior to another. Growing up Christian I went to a multitude of Christian holy (read holiday) days. My Christian family celebrated at a very small town consisting of approx. ten city blocks with around 800 people. This town has two churches; Protestant and Lutheran. This community could have easily worshiped their "one true God" together yet never once did in over thirty sermons. Why can the supposed followers of Satan get along without bickering but the followers of God cannot or will not without saying that their particular version is sacrosanct or inviolate?