Despite the title covering a wide range of dates, this was an event that occurred in the span of a single night of which the date cannot be definitively placed. The night was one that occurred in the summer of 1816, where several English writers including Mary Shelly, Lord Byron, and John Polidori, engaged in a friendly competition to see which of them could write the best mystery story. This followed a previous expedition by Mary Shelly where she travelled down the river Rhine and visited many of the places that would appear in the novel that she would eventually write, Frankenstein; or, The Modern Prometheus. Without this event, it is likely that the novel would not have been written.

 https://en.wikipedia.org/wiki/Frankenstein