Oaxaca's southern latitude and high elevation give it a pleasant climate all year, with daytime temperatures ranging between 70-75 F/21-24 C in winter and 82-90 F/28-32 C in summer. The hottest months are April and May.
The summer rainy season usually begins in June and lasts until September or October. There is no optimum time to visit. Winters are cooler and drier with clear skies, but the landscape is barren. If you prefer lush vegetation and don't mind the showers, then the summer rainy season is a good time to go.