Bakersfield has one of the most extreme climates of any major California city. Summers are brutal: temperatures routinely reach 100°F (38°C) in July and August, and the surrounding valley traps heat and agricultural haze. If you are visiting primarily for outdoor activities or walking around downtown, avoid June through September unless you are genuinely heat-tolerant. The California Living Museum and Kern County Museum are both largely outdoors — in summer heat, these become endurance tests rather than pleasant excursions.
Spring (March through May) is the best time to visit. Temperatures are mild, the air is clearer, and the surrounding orchards and wildflower fields are at their most vivid. Autumn (October and November) is a reasonable second choice — temperatures have dropped from summer peaks but the days are still warm and dry. Winters are mild by the standards of most of the US but can bring thick Tule fog, the dense ground fog that regularly reduces visibility on Valley roads to near zero and makes driving hazardous.