Is the Sabbath Saturday or Sunday?

The argument for Sunday observance of the Sabbath which I find most compelling is as follows:

We begin with the premise that the Bible doesn't actually teach that we are to meet on the seventh day of the week. Rather, it teaches that we are to meet on the seventh day (without the qualification "of the week"). The Hebrew Old Testament normally uses the unqualified phrase "the seventh day" to refer to days which were not the seventh day of the month, and which could not possibly have fallen on the same day of the week from year to year (e.g. Exod. 12:15,16; 13:6; Lev. 23:6-8; 13:5,6,27,32,34,51; 14:9,39; Num. 6:9; 7:1-48; 19:12,19; 28:17-25; 31:19,24; Deut. 16:1-8; Josh. 6:4,15; Judg. 14:12-18; 2 Sam. 12:18; 1 Kgs. 20:29; Est. 1:10). These uses of the phrase (by far the majority of its uses in the Old Testament) appear simply to mean "seven days later." Moreover, there were some Sabbaths that were not on the seventh day (e.g. Lev. 23:27-32:39)

Similarly, in Exodus 16 where God commanded the Israelites to observe the Sabbath in the wilderness, he indicated which day would be the Sabbath not by referring to days of the week, but by telling them to count the days on which they received manna (Exod. 16:4-5,22-23). The Bible does not say that they began to receive manna on the first day of the week, but marks the time from "the fifteenth day of the second month after their departure from Egypt" (Exod. 16:1). We do not possess an ancient Hebrew calendar, and we do not know on which day the Sabbath fell in the wilderness. In fact, it is entirely possible that the Hebrews did not determine which day the Sabbath was by looking at the calendar to find the seventh day of the week, but instead determined the seventh day of the week by first determining when the Sabbath was.

Assessing all the occurences of "seventh day" in the Old Testament, and looking at the institution of the Sabbath, it seems that the Old Testament does not clearly teach that the Sabbath is to be observed on the seventh day of the week -- or at least that the calendar week is not to be used to determine which day is the seventh.

In the New Testament, the Jews celebrated the Sabbath on what was generally recognized as the seventh day of the week (Matt. 28:1; Mark. 16:1-2; Luke 23:56-24:1), and Jesus recognized their choice of days (e.g. Matt. 12:1-13; Luke 13:14-16). However, neither Jesus nor any other New Testament writer indicated that the Sabbath day always had to fall on the seventh day of the week as determined by any regular calendar.

Based on this thinking, the New Testament church, under the guidance of the Apostles, apparently felt the freedom to change the day of observance relative to the secular calendar. They still maintained the commanded six-day-plus-one pattern, but shifted their Sabbath observance to the first day of the week relative to the secular calendar. They chose this day most probably because it was the day on which Jesus had been raised from the dead (Matt. 28:1; Mark 16:2; Luke 25:1; John 20:1). The risen Lord also chose the first day of the week on which to manifest himself to his disciples when they were gathered together (John 20:19,26). In any event, it seems that the first day of the week probably came to be known as the "Lord's Day" (Rev. 1:10), and seems to have been the day on which the church gathered with the approval of the Apostles (Acts 20:7). There does not appear to be any evidence in the New Testament that the early church felt compelled to observe the seventh day of the week as the Sabbath, and there is some possible evidence that Paul taught that Christians were not obligated to observe that particular day (Col. 2:16).

In conclusion, the practice of Sunday observance is based first on the understanding that the Bible does not command observance on the seventh day of the calendar week, and second on church tradition established under the approval of the Apostles.