The celebrations of the Events of the life of Jesus Christ - Epiphany, Christmas, Easter etc - were not instituted at the very beginning of the Christian era. They were held by the early Christians "as vivid commemorations without a connection with certain days and hymns, but as a real Event of the Lord Who was present in the Church". (cf Greek Orthodox of America website). But later, when the Church was firmly established, Christianity no longer a persecuted faith and its believers were free to worship, the Church was able to commemorate and observe annually in the calendar year the events of the life of the Church and especially those of the Life and Person of Jesus Christ, whom they worshipped along with the Father and the Holy Spirit. Therefore special prayers, hymns, and readings appropriate for the occasion were compiled and used to emphasis each aspect of Jesus' life and revelation. The reason why the first Christians did not institute these celebrations and observances at the very beginning is mainly because of the persecutions of the Church and its believers.
It was also important - as the Old Testament feasts prove - to have annual aide-de-memoirs to help not only teach the main tenets of the faith but also to remember them.