Passover Dates

The Passover didn't only have to be observed in the month of Nisan - but it was also fundamental to the meaning of the Passover that it had to be Spring when they kept it. That's one reason the date of Passover isn't the same every year - it has to coincide with the full moon, in the month Nisan, and with the first shoots of Spring. That unique combination of requirements ought to be enough to tell you the Passover was never intended for observance in the Southern Hemisphere (where it's Autumn not Spring in the month of Nisan).