The fourteenth day of every lunar month or the day before the new moon is known as Shivaratri. Among all the twelve Shivaratris that occur in a calendar year, Mahashivratri, the one that occurs in February-March is of the most spiritual significance.
There is more than one story behind celebrating Shivaratri:-
1. One is that Lord Shiva married Parvati on this day. So, it is a celebration of this sacred union.
2. Another is that when the Gods and demons churned the ocean together to obtain ambrosia, a pot of poison emerged. The posion posed a threat to the world. Hence Lord Shiva consumed this poison in order to save both Gods and mankind. Goddess Parvathi hold over his neck so that he don’t gulp down the poison, turning him into blue. Hence named ‘Neelakantha’. To honor the savior of the world, Shivaratri is celebrated.
3. Also it is believed that Goddess Ganga descended from heaven in full force. Lord Shiva caught her in his matted locks, and released her on to Earth as several streams. This prevented destruction on Earth. As a tribute to Him, the Shivalinga is bathed on this auspicious night.
4. Also, it is believed that the formless God Sadashiv appeared in the form of a Lingodhbhav Moorthi at midnight. Hence, people stay awake all night, offering prayers to the God.
Shivaratri awakens the inner spirit and destroys all evils in the Self. Devotees remain awake throughout the night, performing pujas and prayers, seeking Divine intervention to fight against all evils. Meditation is done in the night with the goal of achieving Self-awareness.
