1st week of April 2010 Week 4 December 2009 Week 4 October 2009 Week 4 May 2009 Week 4 March 2009 Week 4 December 2008 3rd week of October 2008 Week 4 July 2008 Week 4 May 2008 2nd week of March 2008 2nd week of January 2008 Week 4 October 2007 Week 4 April 2007 3rd week of January 2007 2nd week of October 2006 2nd week of July 2006
Based on your website or blog's backlinks, traffic Google assigns a rank for your website. The starts from 1 to 10. The more the page rank, better opportunity to attract advertisers
The only confirmed major update was around 2nd April. The dates posted by TheAlkaline are correct. Any changes since then were very minor updates.