The best time to visit Mesa is October through April, when the Sonoran Desert is at its best: warm, dry days, cool nights, and a landscape that goes from brown to startlingly green and flowering after winter rains. February and March add Cactus League spring training to the mix, making that period the busiest and most festive.
Summer (May through September) is brutal. Temperatures routinely exceed 40°C (104°F) in July and August. If you visit in summer, plan outdoor activities for before 9am or after 6pm, stay hydrated, and appreciate the strange drama of monsoon season — from mid-July through September, afternoon thunderstorms roll in from the Gulf of Mexico and drench the desert in spectacular fashion, occasionally producing flash floods on desert washes.
The Mesa Farmers Market runs October through April. The Commemorative Air Force Museum holds special events and airshows primarily in the cooler months.