We do not determine the date of the Sabbath by looking at the calendar and seeing which day is Saturday (or Friday night). Rather (much like my answer to the Samoa" question) the Sabbath is seven days from the previous Sabbath. We have a living mesorah, tradition, on which day is the Sabbath, which happens to coincide with what it commonly known as Friday night to Saturday night. This is one area that illustrates that the observance the Torah commands of us cannot really be reconstructed from the text alone but requires the living community to transmit the practices to the next generation. Nevertheless, this is also a area where there is a fair amount of collaboration since the outside world has its "traditional" dating systems which are seldom altered, and when they are it is generally discussed, and discussions exist about when our Sabbath is in comparison with those systems. (Incidentally, Christians specifically consider the day they loosely call the Sabbath to be the first day, not the seventh. They are not disagreeing with what day of the week is the 'seventh'. Since the accuracy of this statement has been challenged in the comments try every single answer (currently) hereevery single answer (currently) here for example)