For the most part it was some time in September, 2015. I can not for the life of me remember the circumstances but the show was on (the episode was twilight time for anyone wondering) and I was like "hey this isn't garbage" and then I was like "Coolio I guess I'm a brony now lol" but, tbh, I don't count this because I didn't actually watch the show very much other than season 1 and then got burnt out because I am dumb.
I guess technically June 2018 but, 2015 sounds cooler