For me, it was the day I came across Tony Lambert at the Youth Nationals in Dayton, OH back in 1991. First time I ever saw a two hander which gave me the idea to try it myself. As a one hander, I was averaging 215 with a Blue Hammer. I messed around in practice two handed all the time until one day a friend of mine told me to try it in league. That night I shot 740 and never looked back. Been a two hander ever since.
Also, I could not get to where I am today if it wasn't for my mentor Dannial Cohen of Striking Results Pro Shops in Atlanta. He taught me everything I know about watching ball motion, laying out bowling balls, and the modern game so that I could coach and mentor others.