Worm Moon
The story everybody repeats is that March is when the ground thaws and earthworm casts appear on the soil. The Almanac itself has more recently offered a different explanation — beetle larvae emerging from thawing tree bark and dead wood — which fits the season just as well and does not have the problem the first one has.
Where the name comes from
The problem is that earthworms are not native to most of northern North America. Glaciation wiped them out, and the worms in those soils today are overwhelmingly European species introduced after contact. So a name that is genuinely pre-colonial cannot be about earthworms in the places the attribution usually points to — which is a good demonstration of why these lists deserve more suspicion than they get.
Also called Sap Moon, Crow Moon, Lenten Moon.
Old Farmer's Almanac; earthworm biogeography from USDA Forest Service