The first day of the week (the Lord's day)
Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come. 1 Corinthians 16:2
Early Christians gathered on Sunday, the day Jesus rose from the dead, and Paul has them set aside offerings that day. Our Sabbath on the first day remembers His Resurrection.
