Mud season is a period in rural New England in late winter/early spring when dirt paths such as roads and hiking trails become muddy from melting snow and rain.
Mud season occurs only in places where the ground freezes deeply in winter, is covered by snow, and thaws in spring. Dirt roads and paths become muddy because the deeply frozen ground thaws from the surface down as the air temperature warms above freezing. The snow melts but the frozen lower layers of ground prevent water from percolating into the soil so the surface layers of soil become saturated with water and turn to mud.
It is also characterized by giant puddles on the side of paved roads, from large piles of snow melting, with no place to drain off to.