Summer is by far the busiest season, though things never get as hectic at Sequoia and Kings Canyon as they do at Yosemite. During extremely busy periods, such as July 4, you may experience delays at the entrance gates. If you can make it here only when school is out, try to visit midweek. In winter, heavy snows occasionally cause road closures, and tire chains or four-wheel drive may be required on roads that remain open; trails in the backcountry and in wilderness areas aren't accessible (except on cross-country skis). To avoid these problems, visit between mid-April and late May, or early September to mid-October, when the parks are less busy and the weather usually is hospitable.
The falls at Yosemite are at their most spectacular in May and June. By the end of summer some will have dried up. They begin flowing again in late fall with the first storms, and in winter they may be hung with ice, a dramatic sight. "Spring" wildflowers can bloom late into the summer as you rise in elevation. Snow on the floor of Yosemite Valley is rarely deep, so you can often camp there even in winter (January highs are in the mid-40s, lows in the mid-20s). Tioga Road is usually closed from late October through May or June. Unless you ski or snowshoe in, you can't get to Tuolumne Meadows then. The road from the turnoff for Badger Pass to Glacier Point is not cleared in winter, but it's groomed for cross-country skiing, a 10-mi trek one-way.