Om Namah Shivaya!
Unlike some other bloggers who copied Swami Sivananda's description of the festival (from his book Hindu Fasts and Festivals, available for free from dlshq.org--follow link under my "Go Here" tab) word for word, only altering the order, without giving credit, I will let Swami Sivananda's words speak for themselves:
"The name means 'the night of Shiva.' This is a festival observed in honor of Lord Shiva, who was married to Parvati on this day. People repeat the Panchakshara mantra, Om Namah Shivaya. He who utters the names of Shiva during Shivaratri, with perfect devotion and concentration is freed from all sins. He reaches the abode of Shiva and lives there happily. He is liberated from the wheel of birth and death." ~Swami Sivananda
Hara Hara Mahaadeva Shaambho Kaashi Vishwaanaatha Gange
I call the names of The Great Lord Shiva
Who lives on the Banks of the Ganges at Kashi (From Krishna Das' album Breath of the Heart)