Yosemite has so many different moods. When you visit will usually be determined by what you want to see. So many people venture to Yosemite in August or September and are disappointed to not see the spectacular waterfalls. Whenever you go, you will have to make some sacrifices. You just need to decide what you most want to see or experience and go then. Or just go twice!
Winter brings with it snow. Yes it's California, but it's also the mountains. Yosemite Valley lies at 4,000 feet and does receive quite a bit of snow. The surrounding communities, Oakhurst, El Portal, and Mariposa, rarely see that snow though as they are around 2,000-2,500 feet. Glacier Point and the high country are closed, but crowds are thin, except around holidays. The valley is gorgeous under a fresh blanket of snow. The spray from Yosemite Falls makes a thick coat of ice on the cliff around it at night. During the day, you can see and hear chucks of ice over 100 feet tall break off and crash their way to the base of the falls in a thunderous roar that echoes through the whole valley. Snowstorms are common and unpredictable. Always carry chains with you as they could be required at any time.
Spring brings new life. By April and May, waterfalls are at their peak, gushing with water. Wild flowers bloom in June. The Mariposa Grove often opens sometime in March or April with Glacier Point and the high country usually opening by the end of May or early June. Crowds start to pick up by late May.
Summer can be hot and dry. Crowds are at their worst, and lodging prices are steep. Waterfalls start to dry up by August. I usually tell people to avoid this season if at all possible.
Autumn is gorgeous as well. Temperatures cool down but are still pleasant. Crowds diminish greatly. The high country and Glacier Point are still open. November starts the rainy season. The only bad thing about Autumn is the waterfalls are down to a trickle or non-existant.