Justin Martyr: "Day of the Sun"

"SUNDAY is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead. For He was crucified on the day before that of Saturn (Saturday); and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration."

~St. Justin Martyr: First Apology, 67.

Justin Martyr: "On the day called Sunday"

“AND on the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits …”

~St. Justin Martyr: First Apology, Ch. 67.