Happy Birthday to you,
Happy Birthday to you,
Happy Birthday, dear Linux,
Happy Birthday to you!
According to at least one source, Linux 0.01 was released on 17 September 1991 (though the version available from ftp.kernel.org has a file date in 1993).
Other sources use 25 August 1991; apparently this is the date when Linus sent his famous email to comp.os.minix, while 17 September is the day that version 0.01 was available via FTP.
At any rate, I'm picking today to announce it :)