Shiv Puran is celebrated on the night of (Maha) vad 4, when Lord Shiv manifested as Shivaling. There are numerous stories admiring the glory of Mahadev (Lord Shiv) in the Puranas.
Glory
Among these stories the Shiv Puran relates a story of Maha Shivratri’s glory.
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." Shivji also blessed the deer which attained a better destiny.
He has been worshipped in Bharat (India) since ancient times. His meditative-postured murti in Mohenjo-daro was discovered by the archaeologists. In the early days his murti or idol was worshipped. But later it was replaced by the Shivaling. It symbolically represents the jyoti-flame of the fire, and not as a phallic symbol, as has been persistently and ignominiously misrepresented by non-Hindu writers since colonial times. The Shvetashvatava Upanishad (1-13) refers to Shivaling upasana-worship.
Two other significant events occurred on Maha Shivaratri; the onset of Dwapar yuga and the manifestation of the 12 Swayambhu Jyotirlingas of Bharat.
The Garud and Skand Puranas cite similar versions, about a king named Sundersenak and an evil hunter named Chand, respectively.
Ritual
On this day, fasting and night vigil - jaagaran, are advocated. Hindus all over the world perform Shiva puja with bili leaves(Aegle marmelos) and milk abhishek. On Mt.Girnar in Saurashtra, a grand mela is held, where people throng to have darshan of sannyasis and mystics.
Bhagwan Swaminarayan has enjoined devotees to celebrate the day by doing Shiva puja and faraar in the Shikshapatri (79) and pujan with Bili leaves (Shik. 149). He has also included Shiv among the five foremost deities of the Hindu Dharma (Shik.84). Additionally, He regards Shiva and Narayan to be equal (Shik.47).
During His time, He blessed Lord Shiv’s murti in the Junagadh mandir, as Siddheshwar Mahadev. Continuing the tradition, HDH Pramukh Swami has blessed the murtis of Shiv-Parvati in the new Shri Swaminarayan Mandir, in Neasden, London, where the festival is celebrated with great ;eacute;clat along with special puja and milk abhishek, every year.
