The Half Dome cables will not be in place over the May 21-23 permit period. Snowpack in the Yosemite high country is currently well above average (134% in the Merced River basin), with the potential for additional cool, wet weather in the next week. As a result, the Half Dome cable route and subdome are covered in deep snow, making trail conditions poor. For the safety of National Park Service employees setting up the cables and for hikers, the cables will not be put up until conditions improve. We will monitor conditions over the next two weeks and issue an expected opening date as soon as possible. If you have a permit for May 21-23, you will receive a refund for the reservation fee. (Learn more about the Half Dome hike. Please note that permits are required to hike to Half Dome on Fridays-Sundays and holidays beginning this year.)
Source:
http://www.nps.gov/yose/planyourvisit/wildcond.htm