there are many versions of why Mahashivratri is celebrated...
some say it was when Shiva Parvati got married,some say it was when Shiva manifested himself in a linga,some say it was when he performed a tandav...
One of them says that Chandra on being saved by Shiva from the Wasting disease on performing the Maha mrintunjay Jap,leading to the formation of the Somnath Ling,Made every tail end of the waning half of the lunar month "Shivratri" and once a year "Mahashivratri",in honour of shiva
here is the article...dont have the link but it was posted in the discussion threadđ
Chandra, the moon-god, disobeyed his father-in-law, Daksha Prajapati. Daksha had given him 27 wives and told him that he should love all wives equally. Chandra, however, preferred only one of them,Rohini. An angry Daksha, therefore cursed Chandra, that he would suffer from the wasting disease. Each day, his luster would wane and eventually he would disappear forever. As a result, the Chandra started to wane. A terrified Chandra did not know what to do. Being a Deva, a sky-god, he turned to his king, Indra, and begged him for help. "The only person you can turn to is Shiva," said Indra, "because he is not a Deva. He is a Maha-Deva, greater than all gods put together."
Chandra went to Prabhas khetra, performed penance of Mahamrtunjaya mantra and prayed to Lord Shiva, at the end of which Lord Shiva appeared before him. He gave Chandra the boon that in a month, he would grow for fifteen days in one half and in the other half he would keep loosing one Kala (shade) per day and decrease in size. Shiva picked Chandra up and gently placed him on his forehead. Instantly, the moon began to wax once again. Daksha had caused Chandra to degenerate; Shiva had helped him regenerate. Chandra realized why Indra had addressed Shiva as Maha-deva. Chandra established the festival of Shiva-ratri each month, at the tail end of the waning half of the lunar month. And once a year, he established the Maha-Shiva-ratri, to mark the end of the winter months and the waxing of the summer seasons.
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