After her marriage to the poet Percy Shelley, the couple went to stay in a lakeside cottage in Switzerland with the poet Lord Byron in the summer of 1816. One rainy night, after reading a German book of ghost stories, Byron suggested that they all write their own horror stories.
Everyone else wrote a story within the next day, but Mary took almost a week. Finally, she wrote an early version of a story about a scientist who brings a dead body to life. She turned the story into a novel, and Frankenstein was published in 1818. She was 21 years old.
You can see the Writer’s Almanac entry for today here.