May 19th in 1780 was known as the “dark day” in Canada and the New England states because darkness fell at 2:00PM. The cause has never been discovered.
The above comes from “The Ontario Book of Days” by Robbins Elliott.
What caused the “dark day”? Come up with something that sounds possible. Test your idea on at least one other person. What holes do they poke in your theory? Can you fix it or does it evolve into something even better? Write a plot outline around the “dark day”. You don’t need to add characters to it just see where the plot takes you.