Maha Shivratri
Maha Shivratri is a Hindu festival celebrated every year in reverence of Lord Shiva. Alternate common names/spellings include Maha Sivaratri, Shivaratri, Sivarathri, and Shivaratri. Shivaratri literally means the great night of Shiva or the night of Shiva. It is celebrated every year on the 13th night/14th day of the Maagha or Phalguna month of the 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.
Legends
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 associated with Samudra Manthan (churning of ocean), a process in which the asuras (demons) and the gods joined hands to churn out amrut(nectar) from the depths of the ocean, using a mountain and a snake as a rope. Among many things that came out, a pot of poison came out of the ocean.this poison was so potent that it has the power to destroy the whole universe.all the devathas (gods)and asuras ran in different directions to save themselves.no one could dare to stop the spreading poison and on the request of gods, Lord Shiva drank the poison. Shocked by His act, Goddess Parvathi just tightened her noose with her hands over the neck of the Lord and hence managed to stop it in His neck. The poison was so potent that it changed the color of His neck to blue. For this reason, Lord Shiva is also called Neelkanta.
Pralaya (the Deluge)
Another version relates that the whole world was facing destruction and Goddess Parvati worshipped her husband Shiva to save it. She prayed for the jivs (living souls) remaining in se –- like particles of gold dust in a lump of wax -- during the long period of pralaya (deluge) night, that they should, upon becoming active again, have His blessings, but only if they worshipped Him just as she did. Her prayer was granted. Parvati named the night for the worship of Ishwar by mortals Maha-Sivaratri, or the great night of Shiva, since Pralaya is brought about by Him.om shiv
The Lord Shiva's Favourite Day
After creation was complete, Parvati asked Lord Shiva which devotees and rituals pleased him the most. The Lord replied that the 13th night of the new moon, during the month of Maagha, is his most favourite day. Parvati repeated these words to her friends, from whom the word spread over all creation.