Nate Anderson at Ars Technica has written about supposedly controversial doodles and controversial lack of doodles.
http://arstechnica.com/media/news/2009/05/how-googles-cute-doodles-became-satans-pawns.ars

the missing link fossil http://3wq6d.tk
Sputnik http://mgdp6.tk
no doodle for Christmas http://zwdcc.tk
no doodle for Ramadan
no doodle for Memorial Day

the same discussion as every year
Google doesn't make doodles for religious events like xmas or ramadan.
If the event is a sad event like memorial day, google doesn't make doodles too.