March 2012, just looked up my email receipt for purchase from Desura, which was where I bought it initially.
Not sure how I heard about it now, possibly from one of the posts on another zomebie fan-site. Have been enjoying zombie films ever since watching Day of the Dead, back in the old days of VHS rental; first zombie related game was probably Zombi on Commodore 64, I love open world type games, and the slightly retro look of Zomboid is appealing to me, as it reminds me a little of Xcom on Amiga, which is no bad thing.
I didn't find Zomboid too engrossing at first, the zombies looked more like aliens with their identical green heads! I did enjoy the Kate and Baldspot tutorial (hope they re-introduce them one day) and still regret killing Kate by accident the first time I tried to make her more comfy with a pillow. Once the game had received a few updates, the zombies had a little more character, and the town really opened up, and from that point I was fully engrossed and have been playing regularly ever since, 500+ hours on Steam now and that's only going to rack up more now vehicles are in-game. Whilst I'm anticipating more updates, I've certainly had my money's worth, and then some.