I should also answer this by saying that, to my mind, "pro" isn't really a "wow I made it!" moment, especially when you're talking about short fiction (a first sale that's a novel might feel different, I don't know). I think you usually don't realize when that moment was until it's several years in the past.

The day I got the letter in the mailbox (and I still remember it clear as yesterday) I didn't feel like a pro. I think that came when I started thinking about where I should send a particular story, balancing money, exposure, editorial feedback and personal preference. That, to me, was my first real 'pro' moment. Everything before that was "wow, I sold a story!"