There are astronomical events that are truly rare. Let’s start with Great Comets. There are plenty of comets zooming around our Solar System all the time, but most of them never get very bright from Earth. In most cases you need a telescope or a good pair of binoculars just to see them. Rarely (on average, about every 10 years), a comet approaches the Sun and Earth closely enough that it becomes exceedingly bright – bright enough to be noticed by people who aren’t looking for it. Some comets even become bright enough to be seen during the day. These very bright comets are informally called Great Comets.
Or consider transits of the Sun by Venus. That’s when the planet Venus appears to pass across the face of the Sun as seen from Earth. Because the planets’ orbits are not exactly in the same plane, Venus usually passes either “above” or “below” the face of the Sun from our perspective. But in a cycle that lasts 243 years, Venus makes two transits eight years apart, followed by a long period of no transits. The last transit of Venus was in June of 2012, and it was the second of an eight-year pair, which means that the next transit of Venus will not happen until December of 2117.
And then you have locally-visible solar eclipses. Solar eclipses happen two to five times every year, but most of them will only be visible in places you don’t live. Unless you want to travel, you might have to wait centuries between two total solar eclipses that are visible from your location.
So yes, there are astronomical events that certainly justify the use of the word “rare”.
And then there are meteor showers.
Meteors happen when bits of rock enter Earth’s atmosphere at incredible speed (because that’s how you move when you’re in outer space), compress the air in front of them to the point that it begins to glow, then burn up from the intense heat. From the ground they look like a rapidly moving point or streak of light that suddenly appears, then vanishes just as quickly. Although they happen all the time all over the world, they’re generally only visible at night in exceptionally clear skies. On an average clear, non-meteor-shower night, you might see one or two meteors per hour.
Occasionally Earth passes through the path of a comet or asteroid that has left behind a trail of debris. When this happens the frequency of meteors increases dramatically: you may see dozens or even hundreds of meteors in a one-hour period. While it can certainly be fun to stay out late watching a meteor shower, they are hardly rare.
See, comets and asteroids follow fairly predictable orbits around the Sun. If a comet’s orbit crosses the orbit of Earth, then every year – like clockwork – Earth passes through a debris trail left behind by that comet. When this happens, we get a meteor shower. Since there are several comets that cross Earth’s orbit and shed debris as they go, there are several meteor showers each year.
The meteor shower to which this meme refers is called the Perseid Meteor Shower, because the meteors appear to be radiating away from an imaginary point in the sky that falls within the constellation of Perseus. The Perseids happen every year over a course of several days centered on August 12, so they’re no more rare than, say, Christmas. Meteor showers in general are actually far more common than once-a-year holidays, because the Perseids aren’t the only show in town. There are more than twenty official meteor showers scattered throughout the year, although only a couple – the Perseids and the Leonids in mid-November, produce enough meteors to make it worth staying up late.
I don’t object to memes that increase public awareness of astronomical events – goodness knows we need more of that – but I do have a problem with classifying an event as rare when that event (or one similar to it) happens several times a year on predictable dates.