This article covers frequently asked questions about travelling and staying in Yosemite, from a climber's point of view.
Summer is too hot in the Valley.
Lots of climbers go to higher elevations, like Tuolumne Meadows, or Lake Tahoe destinations. The winter is the wettest season, and might be too cold, although there might also be some very nice days with excellent conditions in the sun and no crowds. Basically conditions are quite impredictable during that season. In April, the weather is still variable. In 1994, I spent two weeks in the Valley, and was never rained on.