The two-mile Mariposa Grove Road is open from approximately April through November. When open, you can drive to the grove from Yosemite's South Entrance (off of Highway 41). During summer, however, the small parking lot fills frequently, resulting in periodic road closures. So, it's best to take the free Wawona-Mariposa Grove shuttle.
Hiking Yosemite National Park, 2nd (Hiking Guide Series) by Suzanne Swedo
Hiking Yosemite National Park, 2nd (Hiking Guide Series)