I lived, like many folks, in a Nintendo-dominated area, but we had a decent contingent of SMS owners (as evidenced by the local video store offering an extensive library of SMS titles). I played SMS at friends' homes and I remember how this motley crew of gamers were stoked when previews of the MegaDrive (and later, Genesis) started hitting magazines.
Most folks had to wait until XMAS to play Genesis, but one kid carefully opened up his gift, removed the contents, and was playing Genesis unbeknownst to his parents. Ha!
That was the first time I played Genesis. I reckon it must have been around Sept-Oct. 1989 (beginning of the school year) since it was still pleasant weather. Altered Beast.
I didn't regularly go to ToysRUs, so it wasn't until December that I played at a TG-16 kiosk (Blazing Lazers, I think).