The earliest reference we have to the Lord's day, besides the vague comment made in the Book of Revelation (1:10), is in the letter to the Magnesians written by Ignatius of Antioch in A.D. 110:
The Lord's day is the day on which "our life has sprung up again by him." In other words, it is the day of Jesus' resurrection, the first day of the week.
The Letter of Barnabas, written not long afterward, explains further:
Barnabas goes on to say that 'keeping the day' means being pure in heart in all things rather than refraining from working, as the Jews do. Justin Martyr, writing around A.D. 150, concurs with Barnabas:
I have explained on another page that the early Church kept perpetual Sabbath. In other words, they sanctified each and every day by resting in Christ and living holy.
So in what manner did they "keep" the eighth day, which is the equivalent of the first day?
Ignatius has told us that we ought to "observe" the Lord's day, and Barnabas has told us that the early Christians "keep" that day. In what way did they do this?
The key is in Barnabas' comment that they kept the eighth day "with joyfulness."
The first day was the day upon which Christ rose. Therefore it was to be celebrated. Tertullian tells us that it was considered unlawful to fast or kneel on the Lord's Day. Clement adds that a person keeps the Lord's day when he "abandons an evil disposition."
When the Lord's day is not dragged into a controversy over the Sabbath, it is very simple to understand. It was the day upon which Christ rose, and the early church celebrated it by by not kneeling or fasting upon that day. It was also the day of their primary Christian meeting, as Justin Martyr points out:
For further discussion of the Sabbath, you can go to the Sabbath page.