"The Sabbath is no time for personal anxiety or care, for any activity that might dampen the spirit of joy. The Sabbath is no time to remember sins, to confess, to repent or even to pray for relief or anything we might need. It is a day for praise, not a day for petitions. Fasting, mourning, demonstrarions of grief are forbidden...
One must abstain from toil and strain on the seventh day, even from strain in the service of God...
It is a sin to be sad on the Sabbath day. "
Abraham Heschel, The Sabbath
"Eventually, the Christians began to observe the first day (as Justin Martyr called it "the day of the Sun," according to the Roman calendar) not because it was Shabbat (sabbath) but because it was the day upon which Christ rose from the dead, the central teaching of Christians, vindicating his power through his death to forgive the sins of the world.In fact, it was a "new day," which they called "the eighth day," a new day because it was the day after the seventh day that Christ rose, sanctifying it in a special way as the day of the new covenant, just as God sanctified the Sabbath/7th day from the creation of the universe.The 'eighth day' was the day the new testament accounts record that the risen Christ revealed himself to his disciples over a forty day period..."
...and, lifted directly from the Eighth Day Books website (http://www.eighthdaybooks.com/about_us.html):
"The number eight was, for ancient Christianity, the symbol of the Resurrection, for it was on the day after the Sabbath, and so the eighth day, that Christ rose from the tomb. Furthermore, the seven days of the week are the image of the time of this world, and the eighth day of life everlasting. Sunday is the liturgical commemoration of the eighth day, at the same time a memorial of the Resurrection and a prophecy of the world to come …"-J. Danielou, The Bible and the Liturgy
" … an eighth and eternal day, consecrated by the Resurrection of Christ … There we shall rest and see, see and love, love and praise."-St. Augustine, City of God, Book 22, Chapter 30