There are many mythological legends associated with this festival. One legend goes that once the other two of the triads of Hindu Gods – Brahma and Vishnu – were fighting over who was the superior of the two. Horrified at the intensity of the battle, the other Gods asked Shiva to intervene. To make them realize the futility of their fight, Shiva assumed the form of a huge column of fire in between Brahma and Vishnu. Awestruck by its magnitude, they decided to find one end each to establish supremacy over the other. Brahma assumed the form of a swan and went upwards and Vishnu as Varaha went into the earth. But light has no limit and though they searched for thousands of miles, neither could find the end. During his journey upwards, Brahma came across a Ketaki flower wafting down slowly. When asked where she had come from, Ketaki replied that she had been placed as an offering at the top of the fiery column. Unable to find the uppermost limit, Brahma decided to end his search and take the flower as a proof of having reached the top end. This deception angered Shiva and he punished Brahma for telling a lie and cursed him that no one would ever pray to him. The Ketaki flower too was banned from being used as an offering for any worship, as she had testified falsely. Since it was on the 14th day in the dark half of the month of Phalguna that Shiva first manifested himself in the form of a Linga, the day is especially auspicious and is celebrated as Mahashivaratri. Worshipping Shiva on this day is believed to bestow one with happiness and prosperity.

Another legend has it that one night, a hunter, who was Lord Shiva's devotee, lost his way in the jungle. As he was wandering, he heard a tiger growl. Frightened he scurried up the nearest tree which happened to be a bael tree. He spent the night up there, out of reach of the tiger. To keep himself awake, he kept plucking and dropping bael leaves while chanting the name of Lord Shiva. He was unaware that there was a Shiva Lingam beneath the tree and throughout the night he had unknowingly dropped thousands of leaves on the Shiva Lingam. The night-long chanting, fasting and the patience of the hunter pleased Lord Shiva. It is believed that next morning Lord Shiva appeared in front of the hunter and blessed him.

According to yet another legend, during the great mythical churning of the ocean called Samudra Manthan, amongst other things, a pot of poison emerged from the ocean. The Gods and the demons were terrified as it could destroy the entire world. They ran to Shiva for help and in order to protect the world, he drank the deadly poison but held it in his throat instead of swallowing it. This turned his throat blue and since then he came to be known as 'Neelkantha', meaning the blue-throated one. Shivratri is also a celebration of this event which saved the world.

A legend related to the great epic Ramayana says that King Bhagirath once left his kingdom to meditate for the salvation of the souls of his ancestors. He observed a penance for a thousand years, requesting Ganga to come down to the earth from heaven and wash his ancestor's ashes to release them from a curse and allow them to go to heaven. To fulfill his wishes, Ganga descended on the head of Lord Shiva who let the waters of the river reach the earth through his thick matted locks. The bathing of the lingam is said to commemorate this legend.

Shivratri is considered especially auspicious for women. Married women pray for the well being of their husbands and sons, while unmarried women pray for an ideal husband like Shiva, who is the spouse of Kali, Parvati and Durga. 

On the day of Shivratri, a three-tiered platform is built around a fire. The topmost plank represents 'swargaloka' (heaven), the middle one 'antarikshaloka' (space) and the bottom one 'bhuloka' (earth). Eleven 'kalash' or urns, are kept on the 'swargaloka' plank symbolizing the 11 manifestations of the 'Rudra' or destructive Shiva. These are decorated with the leaves of bilva or bael and mango atop with a coconut, representing the head of Shiva, is kept. The uncut shank of the coconut symbolizes his tangled hair and the three dents on the fruit are believed to be his three eyes.

On Mahashivratri, devotees wake up early in the morning and take a bath, if possible in river Ganga. They then offer prayers to Surya, Vishnu and Shiva as a purificatory rite. After wearing fresh clothes, they visit the nearest Shiva temple. Hindu temples across the country are decorated with lights and colorful decorations and people can be seen offering night long prayers to Shiva Lingam. Bael leaves, cold water and milk are offered to the Shiva Lingam as they are believed to be Lord Shiva’s favourites. With reverberating sounds of Shankarji ki Jai, the devotees circumambulate the Lingam, three or seven times and then pour water over it. Some also pour milk and honey. It is then anointed with sandalwood paste and offered wood apple or bael leaves, fruit, milk, sandalwood and jujube fruit (bér). Since Shiva is believed to be very hot tempered, things that have a cooling effect are offered to him.

The worship continues through the whole day and whole night. Jaagran (nightlong vigil) night is also observed, where a large number of devotees sing hymns and devotional songs, in praise of Lord Shiva. In the morning, devotees break their fast with the prasad that was offered to Lord Shiva the night before, after the aarti.

Hindu mythology suggests that by observing vrat on Mahashivratri, one can attain control over the two great forces that afflict man – rajas guna (the quality of passionate activity) and tamas guna (the quality of ignorance). When a devotee observes fast with discipline and spends the entire day at the feet of Lord Shiva, his/her mind is controlled. Moreover, evil feelings like lust, anger and greed are subdued, thereby acquiring control over rajas guna. When he/she stays awake through the night of Mahashivaratri, he/she develops the ability to conquer the evils of tamas guna too.

The most popular Mahashivratri celebrations take place in Ujjain, which is believed to be the place of residence of Lord Shiva. Large processions, carrying the idol of Lord Shiva, weave through the streets, with people thronging to catch a glimpse of the revered Lord. In Kashmir, the festival is held for 15 days. The 13th day is observed as a day of fast followed by a family feast.