So most of us fans know the song, "Disappearance of Hatsune Miku," right? Well, it wasn't just wrote for nothing. Read more if you 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 search 'Hatsune Miku,' it would lead them to no where. Which, in other words, means that you would not be able to search 'Hatsune Miku' on Yahoo!, Google or 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)