But what's the first day of the week, or the last day of the week? You're missing my point entirely.
Leading astronomers know the difference:
"I have always hesitated to suggest breaking the continuity of the week, which without a doubt is the most ancient scientific institution bequeathed to us by antiquity" (Edouard Baillaud, "The Report," p. 52. [Baillaud was Director of the Paris Observatory.]).
"There has been no change in our calendar in past centuries that has affected in any way the cycle of the week" (James Robertson, personal letter, dated March 12, 1932. [Dr. Robertson was Director of the American Ephemeris, Navy Dept., U.S. Naval Observatory, Washington, D.C.]).
"As far as I know, in the various changes of the Calendar there has been no change in the seven day rota of the week, which has come down from very early times” (F.W. Dyson, personal letter, dated March 4, 1932. [Dr. Dyson was Astronomer Royal, Royal Observatory, Greenwich, London.]).
"The week of seven days has been in use ever since the days of the Mosaic dispensation, and we have no reason for supposing that any irregularities have existed in the succession of weeks and their days from that time to the present" (Dr. W.W. Campbell, Statement. [Dr. Campbell was Director of Lick Observatory, Mt. Hamilton, California.]).
The mother church seems to know "what time it is" also:
"For example, nowhere in the Bible do we find that Christ or the Apostles ordered that the Sabbath be changed from Saturday to Sunday. We have the commandment of God given to Moses to keep holy the Sabbath day, that isthe 7th day of the week, Saturday. Today most Christians keep Sunday because it has been revealed to us by the [Roman Catholic] church outside the Bible."- Catholic Virginian, “To Tell You the Truth," p. 9, Oct. 3, 1947"
"Is Saturday the seventh day according to the Bible and the Ten Commandments? I answer yes. Is Sunday the first day of the week and did the Church change the seventh day—Saturday—for Sunday, the first day? I answer yes. Did Christ change the day? I answer no!"- James Cardinal Gibbons, Archbishop of Baltimore (1877-1921), signed letter
The Protestants know too:
"The festival of Sunday, like all other festivals, was always only a human ordinance, and it was far from the intentions of the apostles to establish a Divine command in this respect, far from them, and from the early apostolic Church, to transfer the laws of the Sabbath to Sunday." - Dr. Augustus Neander, The History of the Christian Religion and Church, Henry John Rose’s translation, p. 186, 1843
"But they err in teaching that Sunday has taken the place of the Old Testament Sabbath and therefore must be kept as the seventh day had to be kept by the children of Israel.... These churches err in their teaching, for Scripture has in no way ordained the first day of the week in place of the Sabbath. There is simply no law in the New Testament to that effect." - John Theodore Mueller, Sabbath or Sunday, pp. 15, 16
It's one day out of seven to rest in God's honor. Which day is first, which day is last? Consider the changes made in the calendar - Gregorian, Julian, European, American, what? We don't even know where days have been added and taken away since biblical times - we've lost track. I mean I guess you could try to figure it out (I'm sure it's been tried before) - but have you? Or are you just taking someone else's word for it?
The conversion from the Julian (created in 31BC) to Gregorian did not break the weekly cycle. Notice how they simply added 11 days from the 4th to the 15th but kept the weekly cycle in tact:
And so we see that the calendar that has been in effect since 31 BC.-prior to the birth of Christ, proves that there has never been any change in the weekly cycle from the time of Christ to now. The Saturday of today is the same Seventh day of the week as it was in Christ's time. We can, therefore, be sure we keep the same Sabbath day Christ kept, setting us an example and confirming the day He claimed to be Lord of (Mark 2:28).