As cited on about.com here is why we celebrate Maha Shivratri,
Maha Shivratri, the night of the worship of Shiva, occurs on the 14th night of the new moon during the dark half of the month of Phalguna (Feb / March) when Hindus offer special prayer to the lord of destruction.
Here’re three reasons to celebrate Shivratri:
1. The absolute formless God, Sadashiv appeared in the form of “Lingodbhav Moorti” exactly at midnight on Maha Shivratri. That is why all Shiva devotees keep vigil during the night of Shivratri and do “Shivlingam abhishekham” (coronation of the phallic idol) at midnight.
2. Lord Shiva was married to Devi Parvati on Shivratri. Remember Shiva minus Parvati is pure ‘Nirgun Brahman’. With his illusive power, (Maya, Parvati) He becomes the “Sagun Brahman” for the purpose of the pious devotion of his devotees.
3. It is also believed that on Shivratri, Lord Shiva became ‘Neelkantham’ or the blue-throated by swallowing the deadly poison that came up during the churning of “Kshir Sagar” or the milky ocean. The poison was so deadly that even a drop in His stomach, which represents the universe, would have annihilated the entire world. Hence, He held it in His neck, which turned blue due to the effect of poison. Shivratri is therefore also a day of thanksgiving to the Lord for protecting us from annihilation.
If you know any other reason why Maha Shivaratri is celebrated? Share in comments.