Apparently, it seems that we are not suppose to sing "Happy Birthday". Well, at least not without paying to the author. It seems that public performance of the song is not allow and is protected by copyright and you need to pay to sing it. In USA, that means until 2030 and EU 2016. Well if it doesn't get extended.
Read about it here:
and remember not to sing this song at parties. Imagine a kindergarden of little children getting busted by a SWAT team and arrested due to copyright infringement. Now, that makes my day.