Rainy Season...Stephen King

This is from Cemetery Dances 2012 calendar

Rainy Season is the thirteenth story in the 1993 short story collection Nightmares and Dreamscapes. It is set in the small town of Willow, Maine, and follows a young couple as they arrive just in time to play their part in a terrifying ritual.
story

John and Elise Graham arrive in the tiny village of Willow, where they plan to spend the summer. (John, presumably a teacher or professor at the University of Missouri, intends to write a book on "the in-migration of the French during the 17th century.") As they make their way to the supermarket, an old man by the name of Henry Eden greets them. He has been expecting them, he says, and tells the couple that they "might want to spend tonight out of town." They are joined by another of the town's residents, Laura Stanton, who reiterates Eden's suggestion, mentions that "rainy season" comes for one night every seven years. Tonight is that night, they say, and Stanton matter-of-factly informs them that this means it will soon be raining toads. They are informed that a room has been reserved for them at a motel in a neighboring town, and Eden tells them that, if they do choose to stay, they will want to close the shutters.

The Grahams, of course, believe that they are the victims of a local joke, and think little else of the toads. That night, however, they are awakened by thuds from the roof and outer walls, as well as the sound of breaking glass. Initially, they believe the residents of Willow are simply trying to frighten them. This thought is quickly discarded as they see the hordes of toads invading the house. Unlike normal toads, these have teeth.

John and Elise retreat to the cellar, and after dispatching the dozen or so toads that accompanied them from upstairs, they barricade the windows with a piece of old plywood. Unfortunately, they neglect to address the abandoned coal chute, and it soon gives way, subjecting them to an avalanche of the carnivorous toads.

As the sun rises, the toads are "evaporated," leaving behind only a cloudy white substance which drains into the earth in short order. Laura Stanton and Henry Eden discuss "the ritual," revealing that "rainy season" always brings a young couple, who are always advised of the danger and always ignore it, and who always meet the same end as the Grahams.