Sunday, the first day of the week, was also referred to as the “eighth day” by Christians.
I will make a beginning of the eighth day, that is, a beginning of another world. For that reason, also, we keep the eighth day with joyfulness, the day also on which Jesus rose again from the dead. Barnabas (c. 70–130, E), 1.147
The eighth day possessed a certain mysterious significance, which the seventh day did not possess. It was promulgated by God through these rites. Justin Martyr (c. 160, E), 1.206
Our Lord Jesus Christ rose from the dead on the first day after the Sabbath. Although the first day after the Sabbath remains the first of all the days, it is nevertheless also called the eighth. Justin Martyr (c. 160, E), 1.215
Righteous Noah . . . with his own wife, his three sons and their wives—being eight in number—were a symbol of the eighth day. Justin Martyr (c. 160, E), 1.268
Concerning the observance of the eighth day in the Jewish circumcision of the flesh, a sacrament was given beforehand in shadow and in usage. But when Christ came, it was fulfilled in truth. For the eighth day (that is, the first day after the Sabbath) was to be that day on which the Lord would rise again, enliven us, and give us the circumcision of the spirit. The eighth day (that is, the first day after the Sabbath), the Lord’s day, was foreshadowed. Cyprian (c. 250, W), 5.354
They foreshadowed the circumcision of the spiritual eighth day. Methodius (c. 290, E), 6.333