The Sierra Nevada foothills are pleasant for most of the year. Spring (March–May) is excellent — the wildflowers are out, the hills are green, and the crowds are minimal. Summer gets hot (regularly over 90°F in July and August) and the surrounding hills brown off, but the caverns provide a cool retreat and the elevation keeps things bearable compared to the Central Valley. Fall is the best season: the crowds from the summer are gone, the weather is warm but not brutal, and the foothill oak woodlands turn gold.
The Calaveras County Fair and Jumping Frog Jubilee takes place every third weekend in May — if this is the reason you're coming, book accommodation well in advance.