It goes away after the second day, though. Your legs are shaking from poor conditioning.
Skiing puts tremendous stress on the quadriceps muscles, especially during long runs.
When a new stress is applied to deconditioned muscles, oxygen debt occurs, lactic acid builds up and the muscles begin to shake. Fortunately, muscles adapt rapidly to the stress and the shaking quickly abates with continued exercise, but, boy, do your legs hurt that evening. In the future, try exercising the quads and other leg muscles in an aerobic fashion. This can include cross-country skiing exercise devices, running, general aerobic classes and any other activity that will condition your leg muscles. I like to roller blade to build up my stamina.