Best time to Visit Yosemite
The best time to visit Yosemite depends on what you want to achieve on that trip.
If you want no surprises in terms of weather and you can deal with the crowds, then Summer (June-Sept) is the way to go.
If you and want to see the waterfalls at their best, then Spring (April-May) is the best choice.
If you would like to go downhill skiing, cross-country skiing or want to see Yosemite in its winter glory (as captured in some of Ansel Adams' pictures), then Winter (Dec-March) is the time to go.
If you want lesser crowds, warm and sunny weather, then Fall (Oct-Nov) is the better time to visit Yosemite.
Summer school vacations is when the largest crowds visit Yosemite so it helps to know when the summer vacations begin.
In the US, summer vacation is generally considered to be between Memorial Day (last Monday in May) and Labor Day (first Monday in Sept).
Most public schools are on summer vacation between early to mid-June through mid to late August. Universities on semester schedules generally end classes in mid-May and start again in late August. Universities on quarter schedules generally end classes in mid-June and start in late September.
If forced to choose, go mid to late May or early to mid October for best combination of weather, crowds and activities.