The bane of re-enactment events. If nothing else rain gives everyone a chance to use all those old english swear words they just happened to stumble across while researching.
Not only does rain dampen (pardon the pun) the liveliness of re-enactment, but historically as well. Rain is incredibly difficult to fight in. It lowers the visibility for scouts and archers, makes armor uncomfortable, and slows down cavalry tremendously.
In addition to the problems of battle, rain (especially cold rain) can cause pneumonia, laying an army to waste just as easily as the enemy. Warm rain attracts all manners of diseases.
Into the gunpowder age, rain made firearms impossible to use. Early gunpowder needed to stay dry, or it would either misfire or not fire at all.