There are many legends associated with the festival as to why Maha Shivratri is celebrated -

According to a legend in the Ramayana, King Bhagirath once left his kingdom to meditate for the salvation of the souls of his ancestors. He observed a penance to Brahma for a thousand years, requesting Ganga to come down to earth from heaven. He wanted her to wash over his ancestor's ashes to release them from a curse and allow them to go to heaven. Brahma granted his wish but told him to pray to Shiva, who alone could sustain the weight of her descent.

Accordingly, Ganga descended on Shiva's head, and after meandering through his thick matted locks, reached the earth. According to a modified version, what reached the earth was just sprinkles from his hair. This story is re-enacted by bathing the linga. The love of water, the primary element of life, is also remembered in this ritualistic action.

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.

According to one, during the samudra manthan, a pot of poison emerged from the ocean. This terrified the Gods and demons as the poison was capable of destroying the entire world, and they ran to Shiva for help. To protect the world from its evil effects, Shiva drank the deathly poison but held it in his throat instead of swallowing it. This made his throat turn blue, and he was given the name Neelakantha, the blue-throated one. Shivaratri is the celebration of this event by which Shiva saved the world.

Mahashivaratri is thus not only a ritual but also a cosmic definition of the Hindu universe. It dispels ignorance, emanates the light of knowledge, makes one aware of the universe, ushers in the spring after the cold and dry winter, and invokes the supreme power to take cognizance of the beings that were created by him.