Ask HN: At what point in your startup did you get your eureka moment?

I don't plan on doing any plugs here for my own startup but I want to get feedback from other founders. I've recently had a serious eureka moment with an early idea-stage startup that I'm working on with a friend and until I can do more validation I'm curious what this moment means for me and what it has meant for others.

At what moment did you think "This is it!"? Did this happen before or after funding and/or bootstrapping? What did that moment mean for you? How did it affect you and your business? How did you determine if those were false signals or not?