I do not recall the actual "Mass of the Faithful" taking place on Saturday and being the day when Catholics in the early Church would meet to witness the Holy Sacrifice and partake of it, although the Sunday obligation is something of ecclesiastical law and not of divine law from what I gather is said on the U.S. SSPX website (www.sspx.org). Nonetheless, that does not mean the obligation should suddenly be moved to Thursday, Friday, or some other day besides Sunday. Traditionally, it seems Saturday was the day of a sort of preparation for the Eucharist on the following day of Sunday. The old Jєωιѕн Sabbath on Saturday was a day when Jєωs would gather in the ѕуηαgσgυє and that is origin of the "Mass of the Catechumens." It was a time for simply reading from the Scriptures and teaching and learning them. Christ rose on a Sunday though, and His Resurrection from the tomb is what was especially preached in the time of the Apostles, and so Sunday is the day when there was regular Mass attendance. I believe St. Paul mentions somewhere in one of his epistles about gathering on Sunday. I think gathering on Sunday can also be found in the Acts of the Apostles. Unfortunately. these days many want to make exceptions into rules. The Saturday evening possibility of fulfilling the Sunday obligation is ridiculous of course as being a norm. It is simply a way for Catholics to find an excuse to not make the sacrifice and get up early on Sunday morning to go to Mass.