Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 5. Christian Worship” (Includes an introduction and sections 59-74).
§65. The Order of Public Worship.
Schaff treats the earliest descriptions of Christian worship in their historical order. He first details the account of Pliny the Younger, dating from 109. Christians would assemble on Sunday at dawn. They would sing, pledge their holiness, then go to work. In the evening they would assemble again to eat together (Schaff 2014, Loc. 15079). Justin Martyr, about 140, describes readings from the Bible, prayer, and communion, along with offerings for the poor (Schaff 2014, Loc. 15093). Schaff notes that worship is described in a similar way by Tertullian and in the Apostolic Constitutions.