The Fast of the First Born (Jewish)

It is an ancient and widespread custom for the firstborn to fast on the day before Passover. This commemorates the miracle which spared the firstborn Jewish sons from the plague which struck down the firstborn sons of the Egyptians.

If there are no children, then the oldest member of the household must fast. They base this on the fact that there was no Egyptian household that was spared from the plague, every household in Egypt was struck whether there was a firstborn son or not.