Groundhog Day, a Grain of Salt, and a Grain of Truth

Of course the coming of spring is not foretold by the seeing of a groundhog's shadow. The celebration came from Germany -- which is why its celebration occurs in central Pennsylvania, an area of significant German immigration -- and occurs exactly six weeks before the vernal equinox used to occur (the change is a result of the precession of the equinoxes, that is, the result of the Earth's axis of rotation itself rotating around another axis). So, spring will be six more weeks, no matter what -- that's just when spring is.

At the same time, the emergence of burrowing animals from their hibernation is a marker of the coming of spring. Like so many of our seemingly mystical rituals, there are perfectly natural and well-supported rules of thumb connected with them. We may have to take them with a grain of salt, but that's not to say that there isn't also a grain of truth there.

What other rituals have back-stories that are well-supported?