But here's a fact question: do all Jews evrywhere, worldwide, observe Passover at the same time? Or do they have calendrical issues too?
It’s always 15th day of the month of Nisan, which corresponds to the full moon of Nisan. The day has varied somewhat for reasons related to harvest, but it’s been mathematical for 1000 years or so.
It’s 8 days outside of Israel and 7 inside.
Sometimes it’s the same as one of the Christian calendars, but I don’t know which one.