Here is the Endless OS release history according to my records, reflecting the date that we publish an OSTree update for existing users, which may not exactly line up with the date that images are available for new users to download. As part of the release process, we add a tag to the stable branch of the relevant git repos, so https://github.com/endlessm/eos-desktop/releases (for older versions) and https://github.com/endlessm/gnome-shell/releases (for newer versions) may be of interest to you (though may not exactly align with the dates that the OSTree was released).
3.3.7 Dec 20, 2017
3.3.6 Dec 14, 2017
3.3.5 Dec 11, 2017
3.3.4 Nov 22, 2017
3.3.3 Nov 13, 2017
3.3.2 Oct 30, 2017
3.3.1 Oct 25, 2017
3.3.0 Oct 23, 2017
3.2.7 Oct 17, 2017
3.2.6 Sep 21, 2017
3.2.5 Sep 11, 2017
3.2.4 Aug 4, 2017
3.2.3 Jul 24, 2017
3.2.2 Jul 13, 2017
3.2.1 Jul 9, 2017
3.2.0 Jul 3, 2017
3.1.8 May 22, 2017
3.1.7 May 10, 2017
3.1.6 May 8, 2017
3.1.5 Apr 24, 2017
3.1.4 Apr 10, 2017
3.1.3 Mar 13, 2017
3.1.2 Feb 20, 2017
3.1.1 Jan 30, 2017
3.1.0 Jan 16, 2017
3.0.11 Jan 9, 2017
3.0.10 Dec 27, 2016
3.0.9 Dec 12, 2016
3.0.8 Dec 7, 2016
3.0.7 Nov 21, 2016
3.0.6 Nov 10, 2016
3.0.5 Oct 23, 2016
3.0.4 Oct 2, 2016
3.0.3 Sep 19, 2016
3.0.2 Aug 28, 2016
3.0.1 Aug 18, 2016
3.0.0 Aug 14, 2016
2.6.10 Dec 1, 2016
2.6.9 Oct 24, 2016
2.6.8 Oct 11, 2016
2.6.7 July 22, 2016
2.6.6 July 14, 2016
2.6.5 June 21, 2016
2.6.4 June 7, 2016
2.6.3 May 28, 2016
2.6.2 May 26, 2016
2.6.1 Apr 25, 2016
2.6.0 Apr 11, 2016
2.5.5 Mar 7, 2016
2.5.4 Mar 1, 2016
2.5.3 Feb 16, 2016
2.5.2 Jan 22, 2016
2.5.1 Dec 14, 2015
2.5.0 Nov 23, 2015
2.4.2 Oct 23, 2015
2.4.1 Sep 29, 2015
2.4.0 Aug 21, 2015
2.3.4 Jun 30, 2015
2.3.3 Jun 1, 2015
2.3.2 May 11, 2015
2.3.1 Apr 29, 2015
2.3.0 Apr 15, 2015
2.2.3 Mar 27, 2015
2.2.2 Feb 27, 2015
2.2.1 Jan 26, 2015
2.2.0 Jan 12, 2015
2.1.10 Dec 4, 2014
2.1.9 Nov 8, 2014
2.1.8 Oct 27, 2014
2.1.7 Oct 10, 2014
2.1.6 Sep 19, 2014
2.1.5 Sep 5, 2014
2.1.4 Aug 26, 2014
2.1.3 Aug 12, 2014
2.1.2 Aug 7, 2014
2.1.1 Aug 5, 2014
2.1.0 Jul 18, 2014
I do not have records for releases prior to 2.1. They were for internal use and targeted pilot testing only, and are completely unsupported.