Shivratri

The absolute formless God, Sadashiv appeared in the form of “Lingodbhav Moorti” exactly at midnight on Shivratri. That is why all Shiv devotees keep vigil during the night of Shivratri and do Shivlingam abhishekham at midnight. Also remember God in his manifestation as Vishnu made his appearance as Krishna at Gokul at Midnight, exactly 180 days after Shivratri, commonly known as Janmashtami. Thus, the circle of one year (360 days) is divided into two by these two auspicious days.
Lord Shiva was married to Devi Parvati on Shivratri (different kalpa). 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.
It is also believed that on Shivratri (different kalpa) Lord Shiva became Neelkantham by swallowing the deadly poison that came up from the ocean during the churning of Kshir Sagar. 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.