My brother subscribed to Nintendo Power when it was a monthly magazine. Before R/B were released, they had Pokemon pull-out inserts in the magazine, spread out over several months. The first half was a walkthrough of the game and the other half was a comic depicting the first few episodes of the anime.
Having not played an RPG-style game before, I was intrigued by the game walkthrough, and its top-down map exploration. That honestly piqued my interest more than the Pokemon themselves. Even before getting the game, I was planning out what Pokemon I would capture and use (of course, I didn't stick to that plan once I actually had Pokemon Blue in my Game Boy lol.)
But yeah, it was Nintendo Power sometime in the 1996-1997 timeframe.