Search for "Season YY" site:blogs.salesforce.com

For example, "Winter 14" site:blogs.salesforce.com

Here are the dates for EU1 for the past couple of years.

Winter ’15 - 17 Oct 2014
Summer ’14 - 18 Jul 2014 – delayed release
Spring ’14 - 18 Apr 2014 – delayed release
Winter ’14 - 12 Oct 2013
Summer ’13 - 15 Jun 2013
Spring ’13 - 09 Feb 2013

Or as one of the comments says, you can use archive.org.