Friendly neighborhood Byzantine Catholic chick weighing in.
Christ was resurrected on a Sunday, which, in Eastern Christain circles is sometimes referred to as the Eighth Day, or the beginning of the New Creation.
In the very, very, very early Church, Jewish Christians attended both synagogue on the Sabbath and had Christian prayer/eucharistic services on Sunday. As the Christians were expelled from the synagogues under persecution, the main focus of worship was shifted to Sunday, thus Sunday, for all intents and purposes, became the Christian Sabbath.
But, really, Christ Himself said, “God created the Sabbath for Man, not Man for the Sabbath.” I really don’t think it matters what day you choose, as long as you have a day of rest, largely set aside for honoring God.
Ever hear of the Orthodox Church? In most countries that have a strong Orthodox tradition, the seventh day of the week is known by some variant of “Sabbaton” (Greek for “Sabbath”). We are supposed to keep the Sabbath holy–although many of us often fail to do so. In addition, we celebrate Lord’s Day (Kyriake in Greek–literally “The Lord’s” or something like that–Sunday in English). Since Christ came in fulfillment of all that had come before, we give it greater honor, although we are supposed to preserve the Sabbath as well. To our discredit, we often fail in that obligation.