I don't know if the following is true or not (since it was taken from a Protestant website), but if it is, how does this square with Vatican II allowing Saturday Masses (for the reason that the Jєωιѕн sabbath starts on Saturday?
There was a dual observance of Saturday and Sunday for a while, but the Council of Laodicea in 364 AD institutionalized Sunday observances with the following edict:
"Christians shall not Judaize and be idle on Saturday, but shall work on that day; but the Lord's Day they shall honor, and as being Christians, shall, if possible, do no work on that day. If however, they are found Judaizing, they shall be shut out from Christ."
Rome later excommunicated any who were keeping the Sabbath on Saturday.