Back long ago, GW released the awesome 2002 calendar. On September 29th it said "Emperor's Birthday". I have never found any other background to support this, but today is as good a date as any. So Happy Bithday to the one who taught us what is best in life, and then conquered most of galaxy. I hope the Custodes put a little birthday hat on him.
The Emperor Protects
Damn, that is a lot of Candles.