The Hebrew calendar was a lunar calendar based on the phases of the moon. A
lunar cycle (today) is about
29.53 days. The Hebrew Passover was at the full moon, or about day 14 or 15 of a 29.5 day cycle.
Obviously it would be hard to get an exact timing, but, over time, one could get fairly close in order to establish religious festivals, etc.
The name of the Hebrew Passover goes back to the Hebrews in slavery in Egypt and the story of the
Exodus or, literally, "
way out", during the full moon. Early Christians referred to "
the way". [Jesus: No one gets there except by me].
What is a full moon?
A crazy guy acting like a cow. Or, better yet (obiter): Why is it called a full moon when you can only see half of it?