The Year without a Summer was 1816. The year before that, in 1815, a volcano had erupted in the Dutch East Indies. The volcano's name was Mount Tambora. The volcanic ash from the eruption spread across the world. It darkened the sky, and made the Earth's surface colder.
In much of Europe and northern North America, summer never came. In New England and Germany, there were frozen lakes in the middle of June. Flowers never bloomed, and every day was cold. These places and China suffered famine.