The date for passover is the first full moon following spring equinox. This is because the Jewish calendar is lunar, so we adjust it every year to conform to the Jewish calendar.
Not quite true. The jewish calendar WAS entirely lunar, and so was Pascha (the proper term for the Catholic Celebration of what in English speaking countries is more commonly referred to as Easter).
However, the Roman empire was strictly aghast at the fact that a holiday could occur twice in one calendar year (The lunar calendar is significantly shorter than the Julian calendar was). Towards that effect, the Christian church had already adopted the Julian calendar as a standard and created a complex calculation to determine what time of the seasons the Pascha occured and to approximate this date so that it would fall on a Sunday within the same reasonable time period as passover would have. This change likewise translated to the Gregorian calendar we follow today.
At a later date, the Jewish church came to accept the standard of a solar year and began to insert “leap months” rather randomly (at first) to make their lunar year conform with the Julian solar calendar. Eventually, this practice became normative and the Jews today follow a complex practice of adding “leap months” on a routine, albeit slightly confusing pattern, which accounts for the occasional difference between our celebration of Pascha (Passover) and the Jewish celebration of passover.
In other words, both the Jews and the Catholics used a complex calculated pattern to transition to the solar calendar, both arriving at different means of doing so, resulting in two different passover events on some years, depending on the calendar year and the jewish leap month system.
