There is an interesting brief discussion at Taanis 27b which says that the Ma'amad would fast four days a week -- but never on Sunday. According to Rabbi Yochanan it is because they were afraid of the Notzrim, aka the "Nazerenes" (who observed Sunday as the Sabbath). I'm not clear whether the Notzrim were proto-Christians that pre-dated Christianity but who had a great influence on the new church, or whether this is simply a reference to the early Christians. Putting that issue to one side, I don't understand why the Ma'amad would be fearful of the Notzrim. If they were Christians, they could not have been an especially large group in Rabbi Yochanan's day, nor would they have been unaware of the fact that even their messiah had observed Saturdays as the Sabbath. Is there more to this story that was censored?