Assembling on the Sabbath: The Ecclesiastical History of Sozomen (c. 425 A.D.)

The people of Constantinople, and almost everywhere, assemble together on the Sabbath, as well as on the first day of the week, which custom is never observed at Rome or at Alexandria.

The ecclesiastical history of Sozomen, comprising a history of the church from A.D. 323 to A.D. 425, bk. 7, ch. 19

(Schaff, Philip, & Wace, Henry (Eds.), A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Edinburgh: T & T Clark, New York: The Christian Literature Company, 1890, vol. 2, p. 390).