Horror is a genre that is known for being non-seasonal or even omni-seasonal. Its viewership is either sporadic or fiercely loyal, and its views depend purely on the discretion and whim of a random viewer. Most would say that October is the prime month to release a horror show, and though this may attract the most viewers, it is not completely accurate. Viewership is based in ease of access, interest/quality, and free time. The largest component of these is free time. Supposing that a horror show has great quality and is accessible (through TV or a streaming service), what matters most is the that the audience has the freedom to comfortably watch. Seasonal releases are a gimmick that can ensure a larger pilot screening, but it cannot ensure success for the post-season.
It's a "strike while the iron is hot" or at least not cold kind of thing. TV shows, even well written and well acted ones have done poorly in the ratings and with advertisers, while crappy shows are on at the right time and do well. AHS seems to still be generating some buzz and some interest. It's managing to survive in the cable age with so many show choices and amidst a wave of new horror shows. Some of that is due to timing and some to the type of show.
Yes, the timing will influence its success, and now may be perfect for it to succeed. For example, there were many clown sightings in the woods on North Carolina recently. Those who have been following the story will be more prone to American Horror Story since it provides an outlet for these real life scary moments.
When fall approaches, many people begin thinking about Halloween. It is an enjoyable tradition for millions of people to watch scary movies or TV shows during the Halloween season. New movies and TV shows with a scary story-line that are released during that time have a higher likelihood of success.