Question: When did sunday became the seventh day of the week?

context: Early Christians continued to pray and rest on the seventh day but also observed Sunday, the day of the week on which Jesus had risen from the dead and on which the Holy Spirit had come to the apostles. Paul the Apostle and the Christians of Troas, for example, gathered on Sunday "to break bread". Soon some Christians were observing only Sunday and not the Sabbath. Patristic writings attest that by the second century, it had become commonplace to celebrate the Eucharist in a corporate day of worship on the first day. A Church Father, Eusebius, stated that for Christians, "the sabbath had been transferred to Sunday".
question: When did sunday became the seventh day of the week and the Sabbath?
short_answers: second century
by the second century
context: The international standard ISO 8601 for representation of dates and times, states that Sunday is the seventh and last day of the week. This method of representing dates and times unambiguously was first published in 1988.
question: When did sunday became the seventh day of the week via ISO 8601?
short_answers: 1988

Answer: Sunday became the seventh day of the week and the Sabbath by the second century, according to Patristic writings. In 1988, the ISO 8601 system was first published, which is an international standard covering date and time to avoid misinterpretation of numeric representations of dates and times. According to the ISO 8601, Sunday is the seventh and last day of the week.