Must've been in late 2011, I guess. I entered November but I'm not really sure. All I know is that it was still 2011 when I first watched it. At the time (and still to this day, really) I spent a lot of time on TV Tropes. More and more often, I came across references to My Little Pony. At the time, I didn't take it very seriously because after all it was freaking My Little Pony.
Then, one day, I was checking Crowning Moments of Awesome of series I remembered from childhood on TV Tropes and once again I saw MLP in there. I remember thinking, how the hay can this series have any awesome moments at all? I read through the list and what I saw truly intrigued me. It didn't come across as a horrible series, that was certain. Being a guy who believes in fair chances (you know, whenever that suits me) I decided to check out the first episode.
At the intro, I remember shaking my head and thinking, 'why am I doing this again?', and that thought became even more prevalent when the intro started. That time, as soon as I heard the opening I muted it, and so I missed the awesomeness that should have given me a clue as to what I was dealing with. My mistake, but obviously long rectified.
When the show proper finally began, I quickly realized that this incarnation of MLP was quite unlike its predecessors. I found myself really liking Twilight and her light sarcasm, especially her reaction to Pinkie Pie. Her reactions were natural, and she seemed as weirded out and even lightly annoyed as I think I would have been in the same situation.
Of course, the opening was a two-parter so I had to watch the second episode as well. This, I think, was the one that cemented the show as a good one, mainly because of Twilight's reaction to Pinkie's song. "Tell me she's not..." That's when I knew beyond a doubt that this show was good, in an entirely different league from older versions. Obviously I immediately went on to watch the rest of the season and by the time I was done, I was completely willing to call myself a brony, eagerly anticipating season 2.
Oh, also worth mentioning as a true turning point for me: Winter Wrap Up. I loved that song the moment I first heard it, and I still love it.
TL;DR: Saw it on Tv Tropes, watched first ep, liked it, watched rest of season.