The seder falls on the fifteenth of Nisan, and the fifteenth of a month that begins at the new crescent is the full moon, every year, without exception.

It is why the escape is traditionally pictured at night and why the Exodus could be made on foot. The Hebrew calendar's major festivals cluster on the fifteenth for the same reason: a full moon is the one night of the month a pre-industrial crowd can move around in. Easter inherits the arrangement.