Myth of Shivratri based on Vishnu and Brahma searching for the origin of Linga
Lord Vishnu and Brahma wanted to know who was superior and this led to a fight. Lord Shiva intervened and said whoever can find out the origin or end of Shivling is superior. Lord Shiva appeared before them in the form of a huge pillar of fire. Lord Vishnu went down searching and Brahma went up searching. Both traveled and traveled but never met the beginning or end.
After the futile search, Lord Vishnu and Brahma prayed to Shiva and appeared before them in the form of Jyotirlinga and this day of the appearance of Lord Shiva is celebrated as Shivratri.
The Story of Shivaratri based on Samudra Manthan
This is a famous legend on Shivaratri and happened during the churning of ocean by Devas and Asuras to get ‘Amrit.’ While churning the ocean, highly toxic poison came out and Lord Vishnu asked the ‘devas’ and ‘asuras’ to approach Lord Shiva. He agreed immediately to help them and drank the poison. In order the poison to have no effect, Lord Shiva should not sleep. So the ‘devas’ and ‘asuras’ kept praying the whole night. Pleased with the devotion Lord Shiva said ‘whoever worships me on this day will get their wishes fulfilled.’
The story of Mahashivratri and the fall of ketaki flower
This myth is similar to the appearance of the Jyotirlinga legend. Brahma went up searching for the end of the Jyotirlinga and Vishnu went down. Brahma after traveling for a while saw a ketaki flower (screw pine) dangling down. He stopped his search and took the flower and returned to Lord Shiva. Vishnu too came back soon and expressed his inability to find the beginning. But Brahma said he found the ketaki flower atop the Jyotirlinga and ketki supported it. Lord Shiva became furious and cursed ketki flower that it will not be offered in worship.
The story of Shivratri based on hunter unknowingly dropping Bilva leaves on Lingam
There once lived a tribal hunter who was a Shiva devotee. One day he lost his way while hunting and was trapped in the forest at night. Soon wild animals started to gather around him and he climbed a Bel or Bilva tree. In order to keep himself awake, he started plucking Bilva leaves and dropped it down repeating ‘Om Namah Shivaya.’ In the morning, he discovered that he had been dropping the leaves on a Shivling. And the word spread that he was saved by Lord Shiva. People started celebrating the day as Shivratri. The story is mentioned in Mahabharata by Bhismha while on the bed of arrows. The hunter was born as King Chitra bhanu who could remember his previous births. And he discussed the importance of Shivaratri with a sage.
Apart from these myths, it is said that the reunion of Lord Shiva and Parvati happened on the Shivratri day. Another legend states that Lord Shiva performed the Taandava on this day.