March through May is ideal: temperatures are pleasant (20–28°C), the citrus groves are in blossom, and the Sunkist Orange Blossom Festival takes place in spring. October through December is the other prime window — fall brings cooler air and the Mission Inn's Festival of Lights runs from mid-November through early January, one of the best Christmas light displays in Southern California.
Summer (June through September) is genuinely brutal, with temperatures regularly reaching 37–40°C. July and August in particular are uncomfortable for walking around outdoors. If you must come in summer, plan indoor activities and move between air-conditioned spaces; the botanical garden and outdoor park visits are better left for other seasons.