Shivaratri is celebrated as birth-night of Lord Shiva.

Sahivaratri is the night of Falgun Krishna Chaturdasi under Hindu Lunar Calendar.  Shivaratiri usually celebrate in month of March.

The Lingodbhav Moorti of lord Shiva was self-immerged at midnight of Shivaratri. So, this night is celebrated as the Birth-night of lord Shiva. Lord Shiva is called Bholenath too.

There are many stories behind the celebration of Shivaratri. 

During Samundra Manthan, the poison Halahal came out of the ocean. It started creating destruction all over. To protect the universe from the devastation Lord Shiva drank the poison and managed to keep it in his throat. His throat turned blue. Since then he is known as Nilkantha. Nila Kantha in Sanskrit is one with blue throat.

Shiva protected the world from the dangerous poison; therefore, people started celebrating the day as Shivaratri and thanking Shiva for shaving the world.

It is believed Lord Shiva married Goddess Parvati on this day.  Some places, Shivaratri is observed as marriage anniversary of Shiva-Parvati too.