Plenty of Christians kept the sabbath throughout history. Well into the 4th century it was apparently so prevelent that the early Roman church had to issue an edict at the council of Laodecia NOT to observe it by resting. Here is Canon XXIX from that council:
CANON XXIX.
CHRISTIANS must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ.
The mere fact that they had to tell people not to observe the sabbath by "resting" over 300 years after the death of Christ shows that many many Christians must have been keeping it.
Show me a single source of anyone even suggesting to keep the Sabbath on a Saturday from between 800 and 1500 AD.