(This post should have been posted on Friday, but extreme exhaustion delayed it until today.)
In a fantasy world with active gods, celestial events can have tremendous impact; the amount of story that can be dragged out of a simple event like a stellar conjunction or eclipse is large for a relatively small amount of effort on the part of the GM. Some are harder than others, depending on the shape of the world in your setting. Worlds where celestial bodies literally represent the gods make it easy, but even in ones that mimic the familiar solar system can have important results for interventionist deities.
A simple stellar conjunction is most typical of interest for gods of the night, the stars, or for the things that lurk in the darkness between the stars. Cultists will mark the progression of the stars and events related to whatever comparatively minor forces they serve. Sacrifices to demons or forgotten outer gods can take place when certain conjunctions occur, and that sort of thing is most likely of interest to heroic adventurers looking for a reason to save people from a terrible fate.
Stellar conjunctions can also mark esoteric events like planar conjunctions and gateways to strange places opening. Creatures from other worlds might sneak into the mortal world on such nights, terrain from other lands might overlay the regular terrain, and taking the wrong turn might lead the unwary to places unknown. They can make a great way to inject some strangeness into your game, either temporarily (ending as the conjunction closes) or permanently (the group finds themselves transposed to a new world when they take a wrong turn or have their home territory transposed forever with some other world).
Eclipses are more dramatic and powerful events; a lunar eclipse can be an omen from the gods, a sign of some great celestial struggle, or an intrusion by some outside force as the moons darkens and turns a bloody shade of red. Lunar gods and their priesthoods are the most likely to be involved; seeking omens and casting divinations to seek knowledge during the event, attempting great sacrifices to strengthen their god against the devouring dark, or undertaking a mission to a sacred place where the eclipse will open a way to the god’s domain – or to the domain that the god otherwise keeps sealed and warded away, to keep the world safe.
Solar eclipses, brief and dramatic like nothing else, are the star of the show for celestial fireworks. Sun gods and their dark and jealous siblings go to war briefly, the righteous sun god always winning – until they don’t, and the eclipse lingers as the sibling takes over, forcing heroes into action to save the world from eternal twilight gloom. Eclipses can also be times when creatures of the night become active for a short while – vampires grabbing victims in the gloom , shapeshifters like werewolves unexpectedly forced to transform by the power of the eclipse, and so on. Omens, portents, prophecies, and childrens with dramatic destinies can also result from solar eclipses, some with a stronger impact than others on a campaign world.
Other events – supernova explosions in the sky that leave a remnant bright enough to see in the day, comets, and shooting stars – can all also have an impact on the world while they linger. Falling stars are often seen as a source of exotic metals like mithral and adamantine, comets are seen as harbingers of disaster, and the flash of a supernova can mean many things – the death or birth of a god, the end of an age, the shift of the balance of the world, or the escape of an ancient evil.
GMs looking to inject a little awesome wonder into their game could certainly do worse than using a few of these in their games to signify events to come.