This year, Mahashivratri is on 21st February 2012 (Monday).

On the occasion of Maha Shiv Ratri, we present before you a quick summary of Shiv-Ratri.

Mahashivratri is a Hindu festival celebrated every year in reverence of Lord Shiva. Other alternate common names / spellings include Maha Sivratri, Shivratri, Sivathri, Shivaratra, Shivratri literally means the great night of Shiva or the night of Shiva.
It is celeberated every year on 13th Night / 14th Day of the Maagha or Phalguna month of Hindu calendar.
Since many different calendars are followed by various ethno-linguistic groups of India, the month and the Tithi name are not uniform all over India. Celebrated in the dark fortnight or Krishna Paksha(waning moon) of the month of Maagha according to the Shalivahana or Gujarati Vikrama or Phalguna according to the Vikrama era.
The festival is principally celebrated by offerings of Bael or Bilva/Vilvam leaves to Lord Shiva,all day fasting and an all night long vigil. In accordance with scriptural and discipleship traditions, penances are performed in order to gain boons in the practice of Yoga and meditation, in order to reach life’s summum bonum steadily and swiftly. A week long International Mandi Shivratri Fair held at Mandi in the Indian state of Himachal Pradesh every year is one of the major tourist attractions in the state.[
According to a legend, Parvati performed tapas, prayed and meditated on this day to ward off any evil that might befall her husband on the moonless night. Since then, Mahashivaratri is believed to be an auspicious occasion for women to pray for the well-being of their husbands and sons. An unmarried woman prays for a husband like Shiva, who is considered to be the ideal husband.
Another legend of the day is that Shiva and Shakti married each other.

There is one more legend of shivratri associated with Samudra Manthan, a battle between devas and asuras during which a pot of poison came out of the ocean and on the request of gods, Lord Shiva drank the poison. The poison was so potent that it changed the color of His neck to blue. For this reason, Lord Shiva is also called Neelkanth.
The Mrtyunjaya-maha-mantra, which is found in the Mahānārāyaṇopaniṣat and chanted as part of ŚrīRudram, is said to eradicate diseases, pain, sufferings and death:

Om Tryambakam Yajaamahe SugandhiM Pushti-vardhanam Urvaarukamiva Bandhanaan- Mrityor-muksheeyamaamRtaat – Om (for chanting Purpose only)

Om. O three-eyed one (Lord Shiva), we worship you, the One of sublime fragrance who is the source of all vitality, growth and splendour. Just as a ripe cucumber is plucked from its vine, May we transcend death and gain Immortality.

A procedure, Trayambak Homa, is associated with this. It consists of an offering of drops of cow ghee on a fire, in a havan kund (sacrificial fire-pot), that uses ghee soaked cow dung as fuel. During this procedure, a prayer is said, a swaha:

Swaha Om Tryambakam Yajaamahe SugandhiM Pushti-vardhanam Urvaarukamiva Bandhanaan- Mrityor-muksheeyamaamRtaat – swaha

Om Tryambakam Yajaamahe SugandhiM sadhana-vardhanam Urvaarukamiva Yonani- Mrityor-muksheeyamaamRtaat – swaha

Om. O three-eyed one (Lord Shiva), we worship you, the One of sublime fragrance who is the source of all vitality, growth and splendour. Just as your ripe cucumber is grows in its vine, May we recieve the shower of vitality and immortality.

May Lord Shiva bless you with success and prosperity in life.

Jai Mahakaal!