According to Hindu Puranas, during the great mythical churning of the ocean called Samudra Manthan, a pot of poison emerged from the ocean. The gods and the demons were terrified as it could destroy the entire world. When they ran to Shiva for help, he in order to protect the world, 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 'Nilkantha', the blue-throated one. Shivratri celebrates this event by which Shiva saved the world.
Maha Shivratri, the night of the worship of Lord Shiva, occurs on the 14th night of the new moon during the dark half of the month of Phalguna. It falls on a moonless February/March night, when Hindus offer special prayer to the lord of destruction. Shivratri (Sanskrit 'ratri' = night) is the night when he is said to have performed the Tandava Nritya or the dance of primordial creation, preservation and destruction. The festival is observed for one day and one night only. This years Maha Shivratri is being celebrated today(24th February 2017).