2019 - Feb 5 ~ Feb 12  - Lunar Year

2018 - Dec 21 ~ Jan 3  - Winter Sale
2018 - Nov 22 ~ Nov 28 - Autumn Sale
2018 - Oct 29 ~ Nov 1  - Holloween Sale
2018 - Jun 22 ~ Jul 6  - Summer Sale

2017 - Dec 22 ~ Jan 5  - Winter Sale
2017 - Nov 23 ~ Nov 29 - Autumn Sale
2017 - Oct 27 ~ Nov 1  - Holloween Sale
2017 - Jun 23 ~ Jul 6  - Summer Sale

2016 - Dec 23 ~ Jan 2  - Winter Sale
2016 - Nov 23 ~ Nov 29 - Autumn Sale
2016 - Oct 28 ~ Nov 1  - Holloween Sale
2016 - Jun 23 ~ Jul 4  - Summer Sale

2015 - Dec 22 ~ Jan 4  - Winter Sale
2015 - Nov 26 ~ Dec 2  - Autumn Sale
2015 - Oct 29 ~ Nov 02 - Holloween Sale
2015 - Jun 12 ~ Jun 21 - Summer Sale

2014 - Dec 18 ~ Jan 2  - Holiday Sale
2014 - Nov 26 ~ Dec 2  - Exploration Sale
2014 - Oct 30 ~ Nov 3  - Halloween Sale
2014 - Jun 19 ~ Jun 30 - Summer Sale

2013 - Dec 19 ~ Dec 2  - Holiday Sale
2013 - Nov 27 ~ Dec 3  - Autumn Sale
2013 - Oct 29 ~ Nov 1  - Halloween Sale
2013 - Jul 11 ~ Jul 22 - Summer Sale

2012 - Dec 20 ~ Jan 5  - Holiday Sale
2012 - Nov 21 ~ Nov 26 - Autumn Sale
2012 - Oct 29 ~ Nov 1  - Halloween Sale
2012 - Jul 13 ~ Jul 23 - Summer Sale

2011 - Dec 19 ~ Jan 1  - Holiday Sale
2011 - Nov 23 ~ Nov 27 - Autumn Sale    /1st big autumn sale
2011 - Oct 27 ~ Oct 31 - Halloween Sale
2011 - Jun 30 ~ Jul 10 - Summer Sale

2010 - Dec 20 ~ Jan 2  - Holiday Sale
2010 - Oct 28 ~ Nov 1  - Halloween Sale
2010 - Jun 24 ~ Jul 4  - Summer Sale    /1st big summer sale

2009 - Dec 23 ~ Jan 3  - Holiday Sale

2008 - Dec 26 ~ Jam 2  - Holiday Sale   /1st big winter sale



ref:
 http://forums.steampowered.com/forums/showthread.php?t=2130715
 https://redd.it/vnrr5

 http://store.steampowered.com/news/?feed=steam_announce&headlines=1
 CTRL+F and "sale"...


 http://steamsales.rhekua.com  (unofficial)