The moment you decide to stop screwing around and just do it already.
The moment you realize it's harder than you thought it would be.
The moment you think you suck so bad at this and why (WHY) are you doing this?
The moment you think you are the single most talented creature who ever touched a keyboard in the world.
Then, the moment you complete your first manuscript.
I expect there are many greater moments to come, (getting published, per say) but I'm wholly convinced that nothing beats the feeling of accomplishing something only you could do. Whether or not you get published or sign an agent or reach the NYT Bestseller status all depend on the actions of others in some way. Finishing that first novel depends on you and you alone.
Yesterday, I walked on clouds for every blinking minute after 4:00pm because I had finally, for the first time in my life, declared my novel done and believed it.
So, after a full twenty-four hours of doing the "it's done" happy dance, as well as re-read (x10) some overwhelmingly wonderful reviews from beta readers, I'm ready to buckle down and pony up to the query letters.
Okay, maybe I'll soak in my euphoria for just one more minute. I don't want to let this feeling go.