14 DAY TRIAL // You know the phrase, it means seldom. Rare. The original meaning of the word was actually "never", but the times change and so do the meanings of words and, in our case, phrases. But the way the astronomical possibility of an event is measured as frequency clearly outdoes my science classes and training.
Oh, you don't know what this is all about, that's why you're pulling the long face. Look at the screenshot or search yourself for "once in a blue moon" in lower case to see this for yourself. The exact value is "once in a blue moon = 1.16699016 ? 10-8 hertz", according to the Google Search Calculator. Is this another Easter Egg, as Haochi of googlified.com names it, or just a gross mistake?
I would go for the first, it's just not possible to go this much out of your way to foul a product that otherwise runs just fine. Or, is it the revenge of an angry engineer that was fired and with his last seconds changed the search line to come up with the result? They have been known to do such things, the engineers. Wondrous beasts, they are indeed.
The first link that you see in the screenshot gives us the meaning of the phrase changed throughout the years: "On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years." The good news is that you will get to live to see this happen more than once in your lifetime, it's not as rare as a comet or as a total sun eclipse. The bad news is that 2007 had one, so you'll have some waiting to do before you'll get to see it for yourself.