But as Christianity gained non-Jewish adherents, some of the communities agitated for separating out Easter from Passover. E., the Council of Christian bishops at Nicea set the annual date of Easter to fluctuate, to fall on the first Sunday after the first full moon occurring on or next after the first day of spring (vernal equinox).
That was intentionally complex because to avoid ever falling on the Jewish Sabbath, Easter's date had to be based on the human week (Sunday), the lunar cycle (full moon) and the solar cycle (vernal equinox).
In antiquity, regnal years were counted from the accession of a monarch. Dionysius is the one credited with the selection of the "AD 1" birth date that we use today—although it turns out he was off by some four years. These men labored to discover what year Christ would have been born by using available chronologies, astronomical calculations, and astrological speculation. E., the Scythian monk Dionysius Exiguus used the earlier computations, plus additional stories from religious elders, to form a timeline for Christ's life.For better or worse, the Gregorian calendar with its embedded Christian timeline and mythology is (essentially) what is used in the western world today.A calendar era is the year numbering system used by a calendar.