The Triduum, the three-day liturgy of Easter — from Holy Thursday evening through Easter Sunday — has been so common an experience of the Christian liturgical year that it is difficult to imagine a time when the Triduum was not. But for at least the first three centuries of Christian worship, this annual celebration of Easter was only one rite, a single grand annual assembly of confessors, and soon-to-be confessors, embracing the life of God incarnate in Jesus Christ and in the members of the community. The theology of this unitive rite took in all aspects of the redemption wrought in the human life of the Son of God, from his conception to his death and resurrection. The whole incarnate life of Jesus of Nazareth was celebrated and entwined in the one liturgy embracing the entire paschal mystery. Though historians have for some time recognized the emergence of the Triduum in the fourth century as a historical phenomenon, the theological and christological contexts of this major liturgical shift of antiquity have never been directly addressed. What theological need, one might ask, prompted the emergence of the Triduum? What void did the paschal mystery fill in its newly extended three-day annual celebration that had been absent in the unitive paschal rite at the earliest stratum of Christian worship?
Connell, Martin F. “Heresy and Heortology in the Early Church: Arianism and the Emergence of the Triduum.” Worship 72, no. 2 (March 1998): 117-140.