The Worm Moon is the full moon which occurs closest to the spring equinox in March. Thus, it is either the last moon of late winter or the first moon of early spring, depending on which side of the vernal equinox the full moon falls.
Native Americans belonging to the Algonquin tribes of the eastern United States and Canada named the Worm Moon for the earthworm castings which start showing up in March, around the same time as the robins return.
The earthworms are active because in this region of the world, the vernal equinox usually marks the time of year when the ground has thawed. There may still be surface freezes, but only at night. Very soon, the freezes will stop altogether, and the ground will be ready for planting.
In England, the Worm Moon is known instead as the Lenten Moon. In the Gregorian calendar, Easter always falls on the first Sunday after the first full moon after the spring equinox, so the Lenten Moon always falls during Holy Week in the last week of Lent, but never on Easter Sunday itself.
Colonial Americans sometimes called it the Sap Moon or Sugar Moon, when the sap in sugar maples starts running and can be tapped to make maple syrup and maple sugar. Maple sugar was particularly valued, not only as a good source of sucrose sugar and needed calories, and even some essential minerals and B-complex vitamins, but also because it had a long shelf life.
However, in most parts of Colonial America south of Maine, the sap in maple trees starts running as early as February, as soon as the daytime temperatures rise consistently above freezing. The best and longest maple sugar seasons occur when the daytime temperatures are above freezing and the nighttime temperatures fall below freezing for a prolonged period of time. Short freezes during this period of time are often called sugar snow.
Other names for the Worm Moon include the Crow Moon, when the sound of cawing marks the return of the crows and the end of winter, and Crust Moon, from the crust the snow forms when the snow thaws by day and freezes again at night. In colder climes, the Worm Moon was known instead as the Death Moon, when the last of the stored winter food runs out and migrating prey animals have not yet returned. The Dakota Sioux, who lived in regions where there was still snow cover at the vernal equinox, creatively called it the Moon When Eyes Are Sore From Bright Snow.
