Mahashivratri is an important day for Hindus.
According to legend, King Bhagirath left his kingdom to meditate for the salvation of his ancestors' souls. He prayed to Lord Brahma for a thousand years, requesting Ganga to come down to earth from heaven and wash away the ashes of his ancestors.
Ganga descended on Lord Shiva's head, as he was the only one who could take her power and force, and after meandering through his thick, matted locks, reached earth.
Mahashivratri, according to this story, is a celebration of this event and is depicted by sprinkling water on the Shivaling.
According to another story, Goddess Parvati meditated on this day to ward off any evil that may befall her husband, Lord Shiva.