Comets come our way fairly regularly, but very few come close enough for people to see the tail without a telescope or the like, so such a comet may only appear in the sky once in a person's lifetime. Sometimes they are even bright enough to be seen in broad daylight, and they can remain visible for weeks or months. It is understandable that people can come to see them as evil omens, and if they got too close they could do as much damage as the asteroid that wiped out the dinosaurs, so they definitely are a potential threat. But they also have been useful to our planet, because they are believed to have been the source of a lot of the water we have in our rivers and oceans. The most famous comet is Halley's, and that comet appeared at the time of the Norman invasion of England at the Battle of Hastings, and is depicted in the Bayeux Tapestry. That would have been a bad omen for one side and a good omen for the other.
Comets come as long-term comets and short-term ones. The former come from the Oort Cloud, about halfway to our nearest star (excluding the Sun) Proxima Centauri and have orbits of thousands of years. The short-term one come from the Kuiper Belt where Pluto lives, and their orbits are counted in hundreds of years. Some, like Halley’s comet, have had their orbits affected by passing by the planets, especially Jupiter.
Conjunctions of planets and other heavenly bodies can be seen as special events. The planets, and the Moon all travel in roughly the same path across the sky, known as the ecliptic, and they take different times to orbit the Sun, with the consequence that every so often, two planets will come close to each other, or the Moon will pass close to a planet, or a planet will come close to a prominent star. Astrologers read special meanings into these events. Some people believe it was a planetary conjunction (or series of them) that became known as the Star of Bethlehem.