If G-d gave the instrunction to Moshe and the nation to keep Shabb-t, He would of-course say when Shabb-t was (even though even before that they knew), and from that time on the over-welming majority kept Shabb-t, so it cant possibly have been confused with monday or sunday. You can also see that all Jews from different parts of the globe all know when Shabb-t is and when wensday is. + each day, the day in counted in the prayers (each day has its own special prayer included).