One Night Or Two Nights

 Passover (Pesach) is on the 15th night of the month of Nisan, which is the night of a full moon. The Hebrew calendar is lunisolar - months go by the phases of the moon around the Earth and years go by the orbit of the Earth around the Sun.

Nowadays, when everyone knows the time wherever they are on the planet, Pesach could be celebrated on one night everywhere. But in the days when people weren't sure what date it was, priests in Jerusalem would judge the date and send out the official news of the night of the new moon to the provinces, and people outside the capital celebrated two nights to be more sure they celebrated on the correct night.