Once upon a time the Devas and Asuras were churning the Samudra Manthan (churning of the ocean of milk) to get amirtham (the nectar of immortality). They used the Vasuki snake , Mount mandara , Kurma ( the Avatar of Lord Vishnu in the form of a turtle ) to churn the samudra manthan. While the churning (Samudra Manthan) happened, many things appeared such as Kamadhenu (wish granting cow) and Airavata (elephant). While they continued, the alakala poison appeared.
The Devas and Asuras were devastated looking at it. Nobody wanted to drink it. At last, Lord Shiva drank it and the poison caused a bluish colouration of His throat. Before the poison spread to His body, Goddess Parvati held Lord Shiva’s neck and stopped the poison at the neck.
From this incident, Lord Shiva was given the name Neelakantan (the blue-throated One) and people in the world celebrated it as Maha Shivaratri.