Yep, You Guessed It: Larsyip Öoldaf Day!
There is no Windows Y. Eagle-eyed readers may already have noted that April Fools Day is an anagram for Larsyip Öoldaf. At least this time it wasn't the Internet Cleaning Day bit.
I expect that approximately nobody was taken in by this bit that occurred to me on the 9th of March and was rewritten several times between then and now.
Microsoft is working on Windows 8. Steve Jobs still wonders Y Windows.