Springtime and early summer are the best times to view the many waterfalls that cascade down the granite walls of Yosemite [1]. You must hike to most of the falls, but Yosemite Falls are visible from the valley floor near Yosemite Lodge.
Actually two separate waterfalls, Yosemite Falls together create one of the highest waterfalls in the world. But both Yosemite Falls are seasonal; if you visit Yosemite Valley [2] during the fall or the winter, you’ll see a trickle of water on the rocks or nothing at all.
The best time to see a serious gush of water is the spring, when the snowmelt swells the river above and creates the beautiful cascade that makes these falls so famous.
Links:
[1] http://www.moon.com/destinations/california/yosemite-and-the-eastern-sierra/yosemite-national-park
[2] http://www.moon.com/destinations/california/yosemite-and-the-eastern-sierra/yosemite-national-park/sights/yosemite-valley