Green rain is a 1.6 weather event that blankets the valley in moss, spawns fiddlehead ferns, and changes animal and villager behavior. Moss harvested during and after green rain is the fastest source of the new moss recipes and the ferns some bundles and gifts want.
This guide explains when green rain can trigger, what to farm during it, which moss nodes respawn, and how to turn the event into resources instead of a visual curiosity.