The Shiv Puran relates a story of Maha Shivaratri's glory. The most popular story associated to MahaShivaratri goes as follows:
A hunter was roaming in the jungle on the bank on the Kolidum River. He was chasing after a deer when he heard the growl of a tiger. He ran as fast as he could and climbed up a tree nearby. The tiger stood at the foot of the tree, and did not leave. All through the night, the hunter had to stay up in the tree. Afraid that he would fall if he fell asleep, he gently plucked one leaf after another from the tree and threw it down.
At the foot of the tree was a Shiva Linga (an image of Lord Shiva). Without realizing it, the hunter, who was sitting on a vilva tree, threw the leaves down at the Linga. The tiger left in at sunrise. The hunter looked down, and found that the tiger was gone, and in its place stood Lord Shiva. The hunter prostrated in front of Shiva and received mukti-the release from the cycle of birth and death.
The popular story of the union of Shiva and his consort, Parvati:
King Daksha, opposed Sati's marriage with Shiva. At a yagnya (holy sacrifice) the king ignored Shiva’s presence and thereby insulted the latter publicly. Sati was so angered by this that she jumped into the sacrificial fire and ended her life. Lord Shiva unleashed his fury at the death of his wife by performing the violent dance, Taandav. He wiped out Daksha’s kingdom, undertook rigorous penance and retired to the Himalayas. The Gods, who feared that the severity of Shiva’s penance might bring an end to the world, revived Sati in the new avatar of Parvati. Shiva-Parvati married and this reunion is celebrated on Maha Shivratri.
There's another popular myth/legend. This story illustrates the greatness of observing the ritual of Shivaratri. Today it is said that whoever fasts all through the night and worships Lord Shiva will attain heavenly bliss:
In ancient times, a Bheel (forest inhabitant) named Gurudruha trudged through a forest to hunt deer. At night, without having sighted a single animal, he unknowingly climbed a bili tree on the banks of a lake. Later at night, a doe arrived to drink water. Gurudruha aimed his bow and arrow at her. While aiming, he unknowingly dropped some bili leaves and his drinking water below on a Shivaling. The deer then requested him to allow her to entrust her fawns to her husband, after which she would return. After much haggling he agreed. While awaiting her return, he stayed awake by aimlessly plucking leaves and dropping them below. Again they fell on the Shivaling. Thus he unknowingly performed its puja while remaining awake all night. Finally the doe returned with her family, She informed him that along with her, he'd have to kill her family too. As he aimed, some more leaves fluttered down on the Shivaling. The collective punya (spiritual merit) accrued from the puja performed unknowingly, eradicated all his sins. This purified his heart. Repenting his flawed life of sin, he set the deer free. As he sat repenting, Lord Shiv manifested in front of him and granted a boon, "You shall be born in a town known as Shrungver, as a man named Gruha. Lord Vishnu will grace your home as Lord Rama and redeem you". Lord Shiva also blessed the deer which attained a better destiny. The Garud and Skand Puranas cite similar versions, about a king named Sundersenak and an evil hunter named Chand, respectively.
MahaShivaratri is celebrated at night, which falls on the 13th or the 14th night of the new moon during Krishna Paksha (dark quarter of the moon) in the Hindu month of Phalgun, when Lord Shiv manifested as Shivalinga (Shivalingam). There have been numerous stories extolling the glory of Mahadeva (Lord Shiva) in the Puranas. He has been worshipped in Bhaarat (India) since ancient times.
Archaeologists have discovered his meditative-postured moorti (idol) in Mohenjo-daro. Initially his moorti was worshipped. Later this was replaced by the Shivalinga, symbolically representing the flame (jyoti) of the fire, and not as a phallic symbol, as has been persistently and ignominiously misrepresented by non-Hindu writers since colonial times. The Shvetaashvatara Upanishad (1-13) refers to Shivalinga upasana-worship.
Two other significant events occurred on MahaShivaratri; the onset of Dwaapara Yuga (yuga = set of a few million Hindu years) and the manifestation of the Dwaadash Swayambhu Jyotirlingam (12 self-born phalluses of fire) of Bhaarata (India).