Frankenstein began as a ghost-story contest on a rainy holiday.
In the cold, wet summer of 1816, Lord Byron challenged his guests at a villa on Lake Geneva to each write a ghost story. An 18-year-old Mary Shelley, after days of having no idea, came up with Frankenstein. Byron's doctor, from the same challenge, wrote the first modern vampire tale.
1816 was the 'Year Without a Summer'. The eruption of Mount Tambora in 1815 had thrown enough ash into the atmosphere to chill the northern hemisphere, and at the Villa Diodati by Lake Geneva the weather kept Lord Byron and his guests indoors. They were Mary Godwin, soon to be Mary Shelley, her lover Percy Bysshe Shelley, her stepsister Claire Clairmont and Byron's physician John Polidori. After reading German ghost stories by the fire, Byron proposed they each write one.
Mary later described being asked every morning whether she had thought of a story, and having to admit she hadn't. Then one night, after a conversation about galvanism and whether a corpse might be reanimated, she lay awake and imagined a scientist horrified by the life he had made. The astronomer Donald Olson has used moon and star data to argue this 'waking dream' happened in the early hours of 16 June 1816.
She meant to write a short tale, but with Percy's encouragement it grew into a novel, much of it written in Bath. Frankenstein; or, The Modern Prometheus appeared anonymously on 1 January 1818, in just 500 copies, when she was 20. Her name first appeared on a French translation in 1821. She revised it heavily in 1831, and that softer version is the one most people read today.
The contest produced a second classic. Byron managed only a fragment about vampires, and Polidori built on it to write The Vampyre (1819), the ancestor of the romantic vampire story. As for the monster: in the book he has no name at all. Calling him 'Frankenstein' is a mix-up that was already being grumbled about in 1908, and the 1931 film sealed it.
Source: Wikipedia — Frankenstein · Text summarised from Wikipedia (CC BY-SA 4.0)