If he scores every period (256 goals a season) then the odds are 1/1 because no matter which period you pick, you're right.
Those are the odds without knowing how many goals in the season he will score. In practice you would have an easy time guessing a random period and being right for a guy like Ovechkin or Kane, because they score a lot more often.
Yeah now I feel pretty dumb. That went way over my head. I guess what I meant was it is 1 of 256 goals not 1/256 chance of him scoring. Therefore if 256 people guessed you'd still have a 1/256 chance of guessing it right and what he did was more impressive then that.