Quite a few stories have rare (or not-so-rare) events that act as plot devices or help shape the story, such as solar eclipses, the phases of the moon, planetary alignments, comets, etc.
While I use a comet in my story, celestial events aren't the only things out there. There are many natural/earthly events you can use, like a plant that only flowers once every century (or something like that).
Anyone else got some?
