So most of us fans know the song, "Disappearance of Hatsune Miku," right? Well, it wasn't just wrote for nothing. Read meer if u want to find out about the story.
A few weeks after Miku was released, her fame had grown to such a large scale that millions of people kept searching "Hatsune Miku" on Yahoo! and Google.
Yahoo! and Google thought that Miku was spam. So when a person would zoek 'Hatsune Miku,' it would lead them to no where. Which, in other words, means that u would not be able to zoek 'Hatsune Miku' on Yahoo!, Google of YouTube.
The fans went crazy nuts, and so the song, "The Disappearance of Hatsune Miku" was created. Seeing the fans' rage because of their action, Yahoo!, Google and YouTube brought back Miku and apologized.
And so, that is why we celebrate 'Disappearance Day' every October. (I don't know the exact date)