From The Old Farmer’s Almanac:
Full Moon Video
Each month, we will explain the traditional names of the full Moon along with some fascinating Moon facts. In this video, learn about the Full Worm Moon, Moon illusion, and when and why the Moon rides high or rides low. Click below to watch video.
Historically, the Native Americans who lived in the area that is now the northern and eastern United States kept track of the seasons by giving distinctive names to the recurring full Moons.
Each full Moon name was applied to the entire lunar month in which it occurred.
The Full Worm Moon was given its name by the Algonquin tribes from New England to Lake Superior. At the time of this spring Moon, the ground begins to soften and earthworm casts reappear, inviting the return of robins. This is also known as the Sap Moon, as it marks the time when maple sap begins to flow and the annual tapping of maple trees begins.
From Western Washington University:
American Indians gave names to each of the full moons to keep track of the passing year. The names are associated with the entire month until the next full moon occurs. Since a lunar month averages 29 days, the dates of the moons change from year to year. Here [is the information from one tribe.]