The Mariposa Grove of giant sequoias, located at the southernmost end of Yosemite, is the largest stand of giant sequoias in the park (with almost 500).
These trees have been around about 70 million years. To put this into perpective, dinosaurs (and many other varieties of living things) went extinct around 66 million years ago, but the giant sequoias survived. Giant sequoias are the largest single organisms ever to live on earth. The oldest giant sequoia is the Grizzly Giant, found in the Mariposa Grove--it is about 2,700 years old?!
The Mariposa Grove is open year-round; however, the road closes with the first significant snow and reopens in late spring (depending on conditions). Most people visit in the summer. If you visit during the winter, bring along your snowshoes or skis! Keep in mind, when the road's closed you'll have to walk about 2 miles to get to the grove.
There are a few ways to enjoy the park. You can walk around the park on your own, which takes around 90 minutes, or go with a resident ranger, who will point out all the sights for you, or go by tram. Trams leave the car park every 20 minutes and take a 1-hour trip around the park with a guide narrating and pointing out interesting trees and relating facts and figures. You can get off the tram whenever you want and catch another tram later at no extra cost. They pass each drop point about every 20 minutes, so you are never left waiting for long. I would definitely recommend the tram tour, as you get to see all the park and learn as you go. Well worth it.
You might be wondering--why would I want to see a lot of big trees? Well, there are trees and there are trees, and these are TREES! It is hard to appreciate just how big and magnificent these are until you stand at the side of one or walk through the base of one. Go there and prepare to be amazed!