For me October is a particularly helpful time of year. One thing I've learned about myself, and it is quite consistent, is I do my best game writing and development during the Fall and especially the month of Halloween. I live next to Salem and that may have something to do with it. Around here Halloween is a huge celebration called Haunted Happenings that brings traffic to a crawl in the surrounding communities. The celebration culminates on the 31st but it builds all month.
New England feels very different in October. The leaves start turning burgundy and yellow and the air grows dry. I always feel like the crisp air sharpens my mind and puts me in a more creative mood. The material I've worked on in October has almost always been the stuff I am most proud of.
Maybe it is because I am at heart a Ravenloft GM and running horror scenarios is something I am most comfortable with. That mood works for me and it pulls me into the game more than any other.
When we first started work on Beneath the Banshee Tree, for example, it was around October of 2013 (in fact I believe this is our first post on the subject: Beneath the Banshee Tree). By the middle or end of October I was running my first play tests and that was by design. Beneath the Banshee Tree is my favorite kind of adventure: a creepy investigation, perfect for Halloween sessions. You can check out a nice overview of it at The Alexandrian.
But even The Patriot Incident, a modern adventure for Terror Network with no supernatural elements at all, was written with an October mood. I don't recall all the precise details of development, but I do now it was written around Halloween, which is why Haunted Happenings in Salem plays such a key role. You can't live where I live and not become pre-occupied with Halloween in October. Even if you hate the holiday, the traffic is everywhere. It is like mardi gras for New England.
When Bill and I did The Landlord's Daughter for Colonial Gothic, I am quite certain that was in October as well.
Sertorius was a multi-year project but once again the times we really hit our stride with material always seemed to be around October. Servants of Gaius was developed over a Summer and into Winter so I don't know if that quite qualifies. Certainly my best Servants of Gaius adventures occurred in the Fall.