The simple answer is that it can't be defined by a single moment. Quitting my job may have seemed like a very assertive, final action that put everything on track, but in actuality it was one action that was flanked on either side by other actions -- some big, some small -- that were just as important as the birthday card/resignation letter saga.
The before... I pursued new relationships; I moved to a new part of town; I gave my boss an ultimatum that if things didn't change, I was going to find something new.
The after... I traveled to new parts of the country; I sat in my kitchen and applied to new jobs; I went to awkward networking events and met more new people.