There are many stories about why we celebrate Maha Shivrathri. Some of the stories are given below.
Lord Shiva appeared as a pillar of fire before Brahma and Vishnu when they fought each other as "who is the greatest". This day is Thirukarthikai. Brahma and Vishu realized their mistake and prayed to Lord Shiva for forgiving their sin. In the night of Shivarathri lord Shiva appeared before them and blessed them.
Long long ago, it is believed that world came to an end and during that dark day, goddess Parvathy Devi did Pooja's and prayed to Lord Shiva to seek his blessings. In remembrance of that day, Maha Shivarathiri is celebrated every year. So the devotees who pray to god like Parvathi Devi on this day will get all happiness when they live and after their life, they will reach lord Shiva and there is no re-birth for them. This is a belief.
On this auspicious day, devotees should chant the following holy names of Lord Shiva to get his blessings
Om Bavaya Namaha
Om Sarvaya Namaha
Om Rudhraya Namaha
Om Paupathaye Namaha
Om Ukraya Namaha
Om Mahadevaya Namaha
Om Bheemaya Namaha
Om Isanaya Namaha
Throughout the night of Shivarathri the Poojas are performed in the Shiva temple. Devotees should fast for the whole day and take part in poojas and pray the god throughout the night by performing abhisheka, chanting and other holy deeds through out the night without sleep. In the early morning they should take a bath and perform an "Annadhanam" and pray Shiva to get gods blessing. Devotees who cannot do a full fasting, can cook Sakkaravelli kizhangu and mix it with sugar and eat once during the day.