Exterior paint doesn't dry so much as it cures, and curing depends on temperature and moisture. Most modern acrylic exterior coatings want a surface temperature between 50°F and 90°F, and they want it to stay above 50°F for several hours after the last coat goes on. In Lancaster County that rules out a lot more days than people expect.
The overnight low matters more than the daytime high. A 62°F afternoon in late October feels like fine painting weather until the temperature drops to 38°F at nine that night and the film stops coalescing halfway through. The paint looks fine the next morning and starts failing the following spring.
Humidity and dew point are the other half of it. Paint applied late in the afternoon on a humid August day can still be soft when dew forms on the siding, which leaves streaks and a weak film. We stop early on those days rather than push one more wall.
That leaves two reliable windows in Central PA: roughly mid-May through late June, and early September through mid-October. Both are cool enough that the coating doesn't skin over before it levels, and warm enough overnight that it cures all the way through.
There's a practical consequence: exterior schedules fill up. If you want your house painted in the fall, the conversation should happen in July, not the week the leaves turn. Estimates are free, they hold for 30 days, and booking early is how you get the weather window instead of the leftovers.