OK, I am in the Midwest, so should be similar to the Northeast. In recent years, I have tried to extend my riding further into the fall, and even winter, or at least until the roads get covered in snow and ice.

So, early September is usually still summer riding weather, but, I start to pay attention as it sometimes is time for a light long sleeve base layer under the jersey on afternoon rides where it is in the high 60s but drops to the mid or low 60s as the sun sets. for temps around 60 to 65 degrees, I switch from lightweight summer base layers to long sleeve but still lightweight baselayers. My personal favorite are Craft and Under Armor. Cooler than 60 degrees, and I switch to a merino wool base layer, long sleeve jersey, knee warmers, merino wool socks, and maybe also pack a light weight windbreaker. And long finger cycling gloves. I also bought a short sleeve thermal base layer over the summer I will try with the long sleeve jersey this fall. Cooler than 50 degrees, and I wear the windbreaker, along with the long sleeve jersey and base layer. I might switch from knee warmers to tights cooler than 50 degrees, and I also might switch from a headband or summer cycling cap to a thermal cycling cap under my helmet. Cooler than 40 degrees, and I add a neoprene shoe covers and switch my gloves to merino wool gloves. I also bought a soft shell cycling jacket which I plan to use this year for rides in the 30s as sometimes my wool baselayer/long sleeve jersey/windbreaker setup still left me a little chilly last year.