Maha Shivratri or Maha Sivaratri or Shivaratri or Sivaratri (Night of Shiva) is a Hindu festival celebrated every year on the 13th night/14th day in the Krishna Paksha of the month Maagha (as per Shalivahana) or Phalguna(as per Vikrama) in the Hindu Calendar. The most significant practices on this day are offerings of Bael (Bilva) leaves to the Lord Shiva, fasting and all night long vigil and worship of Lord Shiva. In North India and Nepal many people consume bhang lassi, which they believe is lord Shiva's favorite drink.
There are many stories associated with Shivaratri and its origins.
Churning of the Ocean
During the samudra manthan by the Gods and demons, haalaa-hala, a poison came out of the ocean. It was so toxic, that its effect would have wiped out the entire creation. At this juncture, as per the advice of Vishnu, the gods approached Mahadev and prayed to him to protect their lives by consuming this poison. Pleased with their prayers, out of compassion for living beings, Lord Shiva drank this poison and held it in his throat by binding it with a snake. The throat became blue due to the poison (Thus Lord Shiva is also know as Neelakantha) and Shiva remained unharmed. This shows that shiva is also the protector. In another story, it is said that the whole world was once facing destruction and the Goddess Parvati worshiped her husband Shiva to save it. She prayed for the Jivas (living souls) remaining in space like particles of gold dust in a lump of wax during that long period of pralaya (deluge) night, should, upon becaming active again and in the enjoyment of their short day and night, have his blessings, but only if they worshiped him just as she did then. Her prayer was accordingly granted. Parvati named the night for the worship of Iswara by mortals Maha-Sivaratri, or the great night of Siva, since pralaya is brought about by him. This period is really his night from the great night or pralaya which was the cause for the origin of this Sivaratri 
After creation was complete, Parvati asked Shiva of which rituals pleased him the most. The Lord replied that the 14th night of the new moon, during the month of Maagha, is my most favourite day. It is known as Shivaratri. Parvati repeated these words to her friends, from whom the word spread over all creation.
Once upon a time, a hunter worshipped Lord Shiva unknowingly on Shivaratri. He did this by dropping bael leaves on a shiva linga at the base of a bael tree from its branches where he was hiding and fasting all night. For this he was forgiven of all his sins. This forms the basis behind the offerings of bael to the Lord on Shivaratri.
The Story Of King Chitrabhanu
In the Shanti Parva of the Mahabharata, Bhishma, whilst resting on the bed of arrows and discoursing on Dharma, refers to the observance of Maha Shivaratri by King Chitrabhanu. The story goes as follows -
Once upon a time King Chitrabhanu of the Ikshvaku dynasty, who ruled over the whole of Jambudvipa, was observing a fast with his wife, it being the day of Maha Shivaratri. The sage Ashtavakra came on a visit to the court of the king.
The sage asked the king the purpose of his observing the fast. King Chitrabhanu explained that he had a gift of remembering the incidents of his previous birth.
The king said to the sage that in his previous he was a hunter in Varanasi and his name was Suswara. His only livelihood was to kill and sell birds and animals. One day while roaming through forests in search of animals he was overtaken by the darkness of night. Unable to return home, he climbed a tree for shelter. It happened to be a Bael tree. He had seen a deer that day but let it live, after seeing the deer's sad family. As hunger and thirst tormented him, he was kept awake throughout the night. His canteen leaked water as he thought of his poor wife and children who were starving and anxiously waiting for his return. To pass away the time that night he engaged himself in plucking the Bael leaves and dropping them down onto the ground.
The next day he returned home and bought some food for himself and his family. The moment he was about to break his fast a stranger came to him, begging for food. He served the food first to stranger and then had his own.
At the time of his death, he saw two messengers of Lord Shiva. They were sent down to conduct his soul to the abode of Lord Shiva. He learnt then for the first time of the great merit he had earned by the unconscious worship of Lord Shiva during the night of Shivaratri. The messengers told him that there was a Lingam at the bottom of the tree. The leaves he dropped fell on the Lingam. His canteen, which leaked water, washed the Lingam and he had fasted all day and all night. Thus, he unconsciously worshiped the Lord.
As the conclusion of the tale the King said that he lived in the abode of the Lord and enjoyed divine bliss for long ages and now he has reborn as Chitrabhanu.