While another legend plays a more prominent role on this day- Gods and Demons are decided the Samudra Manthan. During Samudra Manthan a highly toxic poison (vish) came out of the ocean. Nobody wants to accept this poison then as per the suggestion of Lord Vishnu, gods approach Lord Shiva and prayed him to protect life by consuming this highly toxic poison. Satisfied with their prayers, out of compassion for living beings, Lord Shiva drank this poison and held it in his throat or neck by necessary it with a snake. The throat became blue due to this poison so Lord Shiva is also known as Neelkantha (‘Neel’ means blue and ‘Kantha’ means neck or throat.) .And Shiva remained uninjured. The clever men advised gods to keep Lord Shiva alert throughout the night. To keep him alert, Lord Shiva performed Tandava. Tandava means dance of primeval creation, preservation and devastation. A vigil was thus kept by the gods in observation of Shiva. As the day broke out, Shiva, satisfied with their devotion blessed them all, and also said that whosoever worshipped & contemplate on him on this day shall be blessed with the completion of his or her wishes. In view of the fact that, on this day and night - devotees fast keep vigil, sing glories of Lord and contemplate.
Manifestation Of Lord Shiva As A Jyotirlinga
God Shiva Mahashivratri History - King Chitrabhanu