The time to ski is after the islands get a big front passing through with lots of precipitation from high clouds, and Mauna Kea gets a good dump of snow. Depending on the wind, it may take a few days for the road to be cleared. Hawaii gets a lot more precipitation and so more snow in La Nina winters. El Nino winters can be very dry. The snow can have some very strange texture after days of exposure to the intense sun.
Some runs can be quite steep and the snow can end abruptly.
There is no soft vegetation underneath, only hard lava rock and abrasive cinder. You should be in very good physical shape, aerobically. If you have the Adobe Acrobat Reader installed on your computer, you should read the UH Institute for Astronomy information on having a Safe and Enjoyable Trip to the Summit . While the snow conditions on Mauna Kea may not be the best in the world, the observatories are. You may find Summit Tours of the Onizuka Center for International Astronomy and the Visitor Information Station very interesting.