Justin wrote prior to 165. He is perhaps granted more credit for accuracy than he may deserve, as he like Eusebius, who came later, wrote under the patronage of an emperor (Titus Antoninus Pius).
This is a small portion from what is known as his First Apology.
“But 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 savior 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.”
It is worth noting, contrary to some opinions and teaching otherwise, the Christians are identified to have met on the first day of the week, not the Sabbath.
This is another small piece of one of the many ancient records of Christianity.