There are two high seasons for the Sierra: from mid-October to mid-November and March through April. These months are the most popular because of the likelihood of moderate temperatures -- but even in these months, temperatures in the bottom of the canyon will be warm. Most canyon visitors stay up in the rim country. To avoid the crowds and get cheaper prices, I suggest going in August or September (the rainy season). During that time, barring drought, you'll find occasional afternoon thundershowers (very pretty in the canyon land), green vegetation, flowing water, and comfortable temperatures up along the rim. In the winter, temperatures at the bottom of the canyons are actually moderate. Avoid the Sierra from late April through June. This is the driest part of the year, with chronic water shortages in many of the towns and hotels; the vegetation is brown, and the canyons can be hazy.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.