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The renewal for season 13 made a landmark this Sunday.
 
It was the earliest Supernatural has ever been renewed
 
Season 12 - March 11, 2016
Season 11 - January 11, 2015
Season 10 - February 13, 2014
Season 9 - February 11, 2013
Season 8 - May 3, 2012
Season 7 - April 26, 2011
Season 6 - February 16, 2010
Season 5 - February 24, 2009
Season 4 - March 4, 2008
Season 3 - May 7, 2007 (renewal came at the upfronts)
Season 2 - May 18, 2006.  This was the upfront for the new CW network merging WB and UPN.  They didn’t announce SPN was part of the lineup officially until the upfront, although many thought it would be back.  
 
The CW is getting more assertive about renewing shows early. Perhaps next year we will be renewed as a Christmas present for fans and the show!
Many thanks to Alice for getting this information!