The day signifies the memory of defeating darkness in life and the world. It is on the auspicious day of Maha Shivratri that Lord Shiva and Parvati got married. Since then this day is called Shivratri.
Lord Shiva devotees observe Maha Shivratri by resorting to strict fasting for the entire day. They can only eat fruits and milk on this day as part of the ritual.