Why Saturday is Not Saturday, Nor Sunday Sunday?

This study establishes that the names of the days of the week underwent a specific renaming at a point in history (the fourth century CE), leading to an apparent shift of three days relative to the ancient perspective of naming the days according to the ruling celestial body at the first hour of nightfall in early Rome and Judea. To this end, we present: 1) archaeological evidence (Pompeian graffito from 6 February 60 CE); 2) Babylonian–Greek–Roman astrological origins for the days of the week and their order according to the ruling celestial body at the hour of nightfall; 3) a technical analysis of how the transition in nomenclature occurred; 4) Coptic Paschal letters of Bishop Athanasius, whose tables show the validity of two different parallel nomenclatures even for the days in the mid-fourth century CE; 5) empirical evidence of the chronological displacement anchored in seven historical milestones reaching as far as the second century CE: Jewish history (with Babylonia in 586 BCE and with Greece in 161 BCE) and with Rome (captures of Jerusalem in 63 BCE, 36 BCE, 66 CE, and 70 CE); Roman history and early Christian Church beliefs regarding the date of the death of St. Polycarp of Smyrna, as well as the reasons for the Gnostic discrepancy regarding the Baptism of Christ documented by Clement of Alexandria in the second century CE; 6) the corroboration of an ecclesiastical synchronization in the fourth century with respect to the two systems of names, and the consequent Theodosian resolutions of forced unification that the Roman Empire applied (321–387 CE) to unify the names of the Roman days in the Alexandrian convention down to the present day.