Homeowners in colder parts of the country have a short painting season. In San Diego County we can paint exteriors in almost every month, but that doesn't mean every day is a good painting day. Coatings need the surface to be dry and the temperature to stay within the manufacturer's range while the paint cures.
On the coast, the marine layer is the main issue. May and June mornings can leave stucco damp until late morning, and paint applied to a wet wall won't bond. We start coastal jobs later in the day during those months and give walls time to dry after washing.
Inland, in Valley Center, Escondido, Ramona, and Fallbrook, the concern flips to heat. Painting a west-facing wall at 3 PM in August means the paint dries before it can level, leaving lap marks and a weak film. We follow the shade around the house and paint hot elevations in the morning.
Fall is the sweet spot for most of the county: warm, dry, and stable, with the marine layer gone and the summer heat easing. Spring is a close second. Winter works well too, as long as we can plan around the occasional storm; a rained-on fresh coat is a redo.
The practical answer is that any month works if the painter schedules around the conditions. Ask how your painter handles morning moisture and afternoon heat. If they paint the same way in every season, that's a warning sign.