Yosemite is considered one of the “Big 3” in the US National Parks System (the other two being Yellowstone and the Grand Canyon). Its proximity to San Francisco makes it a popular park, and the works of Ansel Adams has given it a place in the American psyche.
Yosemite should be on everyone’s list of parks to visit at least one time in their lives. If you are not from the United States, then it should be high on your list of things to see if you visit the country.
I’ve been to Yosemite twice: once in May and once in October. I would definitely recommend a spring visit over a fall visit. By the end of the summer, the waterfalls are usually dry and you can’t see the wonder of some of the highest waterfalls in the world. If you visit before Memorial Day, you can also usually avoid the large crowds.