The Bible first mentions the Sabbath in relation to the giving of manna to Israel (Exodus 16:22-30). For six days manna fell in the camp of Israel with twice as much falling on the sixth day. This gave them sufficient food for the seventh day, since no manna fell on this day.
Did the manna fall for the first time on Sunday? If not, they did not observe Saturday as a Sabbath. If manna fell on Monday for the first time, Sunday would be the seventh day for them. The fact is, we do not know what day the manna fell according to our Julian calendar. God also commanded Israel not to go out of their dwelling on the seventh day. If we observe a Sabbath, must we observe this part of the rule?
Apparently there was a time change for Israel when they left Egypt (Exodus 12:1-2). Their departure from Egypt was a new beginning of days and months. No longer were their lives governed by Egyptian time. We should think on these things and how they relate to the observance of a special day.