Om. Parvati Pate. Hara, hara, Mahadev.
Shivaratri is a day of prayer and worship dedicated to lord Shiva. It is the day that Shiva and Parvati were married. Shivaratri occurs each month on the Indian lunar calendar, near the new moon. The most important shivaratri occurs in the month of phalguna, roughly February or March. This is supposed to be the darkest new moon1 and is called Mahashivaratri.
Significance of the new moon
One of the traditional images of Shiva is the god sitting in meditation with a crescent moon in his hair. The moon represents the mind, and the crescent in shiva's long hair symbolizes his control over the mind. A disciplined spiritual seeker has control not only over his or her passions, but also over the chaos of the mind. The time of the new moon is not only one of outside darkness, but of the calming of the mind through meditation and other spiritual practices.
There is a story in the santi parva in the Mahabharata describing the power of worship on shivaratri.
On his deathbed, bishma tells the following story.
There was once a hunter who spent all day hunting. It was getting dark by the time he had killed his prey and he climbed a tree to wait out the night. He did not notice at the time that the tree he climbed was a bilva2 tree. In the twilight a tiger caught wind of him and the deer and paced under the tree. The hunter, afraid, nervously picked leaves from the tree and wept at the thought of not making it home to his family. Hidden among the roots of the tree was a shivalingam, and each leaf and tear that fell, landed on this sacred object. Fearful of the tiger below, the hunter stayed awake all night, unable even to go in search of food or water. By morning the tiger had gone, and the hunter descended and went home. Upon his deathbed many years later, Lord Shiva appeared to him. When the hunter asked how it was he was so honored, Shiva told him of the night he kept vigil in the bilva tree.
“Underneath the tree was a lingam, and all night you fasted, dropped leaves and washed the lingum with your own tears. I was touched by your worship and have come to grant you moksha."3
Observance of shivaratri
Devotees of Lord Shiva observe shivaratri with fasting and prayer. Sometimes the name of Shiva is chanted and a vigil is kept all night4. As the story of the hunter indicates, the shiva lingam is a sacred object of worship on this day, and is ritually washed in milk and/or rosewater. It is said that the power of repeated shiva’s name on this day is increased a hundred-fold.
1I have been unable to locate actual evidence as to whether there is such a thing as one new moon being darker than another. If you know, please /msg me!
2 aegle marmelos a type of fruit tree considered sacred.
3In another version, the tiger is also granted moksha for keeping vigil before an alter of shiva.
A beautiful interpretation of the spiritual significance of this story can be found at: http://www.sivanandadlshq.org/religions/shivaratri.htm#sig
4The mantra Om Namah Shivaya or texts such as the Shiva Mahimna Stotra are often chanted.