This post illustrates why it is wide to read a thread before actually commenting on it.
It does seem to me that if God prescribed the seventh day (working on that assumption for now) I would have to know what the first day is. Here we have a number of problems:
(1) Whose first day.
My calendar says the first day of the week is Sunday. My company says the first day of a work week is Saturday. Psycologically the first day of the week seems to me (working a normal 5 day week) to be Monday. However this is just a cultural convention. What is to prevent our culture from suddenly deciding the first day of the week is Tuesday. The only reason I can think of is there is no need to do this. However, do I know that every culture on the face of the planet has a first day of Sunday.
In other words I do not know whether everybody has a common first day of the week. Furthermore, I have no idea what the first day of the week may be in the past. So the question in my mind is whose first day?
(2) Calendar changes
But I do know that the first day of the week is not the same first day of the week in the time of Christ. The calendar was adjusted I think by 10 or 11 days in the 1700s or so due to various inaccuracies. Does this mean that the first day is now Wednesday or Thursday? Do I know that God gave his church the right to cheat the week this calendar adjustment was made? What if God’s calendar was not adjusted? Then we are all hosed.
The above post also made excellent comments about the time zone problem and the above the arctic circle problem.
The net here is that I begin with the assumption that it is very important that I observe the sabbath as the seventh day of the week, I have a big problem. I don’t know what God’s first day of the week is with absolute certainty, let alone what God’s seventh day is. If our eternal salvation rests on the dependency that we must know with absolute certainty what God’s first day of the week is, then I am afraid we all are in big trouble.