There are many reasons why Siva Ratri is auspicious, but one of the most popular belief comes from the 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.
Today is a fast until noon in observance of the appearance anniversary of Lord Nityananda. Nityananda Prabhu appeared as Chaitanya Mahaprabhu's principal associate for spreading the congregational chanting of the holy names of the Lord. He appeared in 1474 in the village of Ekachakra, now in West Bengal. He especially spread the holy name of Krishna throughout Bengal. His beauty was so enchanting and He was so full of ecstatic love for Krishna that, wherever He moved, crowds of people would follow and become lovers of God. He is considered an incarnation of Krishna's principal expansion, Balarama.
Mahek, Simren, Ishaan, Tulsi, Janki, Sneha, Manushi, and other sources