Why is passover celebrated for 8 days?
I’m sure you’ve noticed that the Torah itself mandates only seven days for Passover (perhaps allowing for a whole cycle of this-worldly affairs found in a week to be affected by the heightened spiritual lessons of the holiday). The eighth day (for those living outside of Israel) is, in part, a product of a geographic condition that existed during the Second Temple period.
Throughout the Second Temple era (from approximately 352 BCE - 68 CE (Common Era), the majority of Jews lived in Persia (Babylonia; modern Iraq).
However, the calendar was set monthly in Israel (based on sightings of each new moon). All of our festivals depended on the decisions made in Israel, but not everyone who lived far away was able to find out about important dates in time for their observance (even though most communities had individuals who could calculate it based on given algorithms and a hugely accurate figure for the mean lunar cycle). Therefore, since there could only be a maximum discrepancy of one day per month, and in order to ensure proper observance even during periods of extreme political unrest when careful calculations would be difficult, we add one day to each festival, just in case.
Since the Fifth Century, a permanent calendar (based on accurate algorithms which diverge from NASA calculations only after four decimal places) has been set independent of visual sightings.
With best regards,
Rabbi Boruch Clinton