I'm sharing and pasting the posted link from Wikipedia about the significance of the Maha Shivaratri.
Hindus celebrate Mahashivratri, because – according to Vedic Literatures – there is a legend associated with Samudra Manthan (churning of ocean), a process in which the asuras (demons) and the devas joined hands to churn out amrut(nectar) from the depths of the ocean using a mountain as a churn-dash and a snake as a rope. The devas'(deities) and the asuras'(demons) counterparts were churning for the nectar of immortality. Amongst many things that came out of the ocean was a pot of poison. This poison was so potent that it had the power to destroy the whole Universe. The poison could not be discarded;it had to be drunk by one of the devas or asuras. Neither wanted to drink the poison because they all felt that they were too valuable or sacred to drink it. Shiva, upon the request of the gods, came forward, and, with a calm disposition, said that he would drink the Halahala (poison) for “the sake of his family(universe) to sustain peace and allow them to find the nectar of immortality.” By drinking the Halahala, Shiva eliminated its destructive capacity. Shocked by his act, Goddess Parvathi strangled Shiva’s neck, hence managing to stop the Halahala from spreading all over the Universe. The poison remained in Shiva's neck, however, and was so potent that it changed the color of His neck to blue. For this reason, Lord Shiva is also called Neelkanta. After drinking the poison, Shiva went to the Himalayas to meditate. Realising that the nectar of immortality was found,the asuras tried to steal it from the devas, as they wanted to become more powerful than the devas in order to be able to destroy them. After a “series of divine interventions,” the devas emerged as the winners and received the gift of immortality. By drinking the poison, Shiva sacrificed himself for the safety of his family(universe). 
Pralaya (the Deluge)
Another version relates that the whole world was facing destruction and Goddess Parvati worshipped her husband Shiva to save it. She prayed for the jivs (living souls) remaining in se –- like particles of gold dust in a lump of wax—during the long period of pralaya (deluge) night, that they should, upon becoming active again. Have His blessings, but only if they worshipped him just as she did. Her prayer was granted. Parvati named the night for the worship of Ishwar by mortals Maha-Sivaratri, or the great night of Shiva, since Pralaya is brought about by Him.om. Shivratri is also when Godess Parviti and Lord Shiva married again
The Lord Shiva's Favourite Day
After creation was complete, Parvati asked Lord Shiva which devotees and rituals pleased him the most. The Lord replied that the 14th night of the new moon, in the dark fortnight during the month of Phalgun, is his most favourite day. Parvati repeated these words to her friends, from whom the word spread over all creation.
The Story Of King Chitrabhanu
Once upon a time King Chitrabhanu of the Ikshvaku dynasty, who ruled over the whole of Jambudvipa (India), was observing a fast with his wife, it being the day of Maha Shivaratri. The sage Ashtavakra came on a visit to the court of the king.
The sage asked the king the purpose of his observing the fast. King Chitrabhanu explained that he had a gift of remembering the incidents of his past birth, and in his previous life he had been a hunter in Varanasi and his name was Suswara. His only livelihood was to kill and sell birds and animals. The day before the new moon, while roaming through forests in search of animals, he saw a deer, but before his arrow flew he noticed the deer's family and the their sadness at its impending death. So he let it live. He had still not caught anything when he was overtaken by nightfall and climbed a tree for shelter. It happened to be a Bael tree. His canteen leaked water, so he was both hungry and thirsty. These two torments kept him awake throughout the night, thinking of his poor wife and children who were starving and anxiously waiting for his return. To pass away the time he engaged himself in plucking the Bael leaves and dropping them down onto the ground.
The next day he returned home and bought some food for himself and his family. The moment he was about to break his fast a stranger came to him, begging for food. He served the food first to stranger and then had his own.
At the time of his death, he saw two messengers of Lord Shiva, sent to conduct his soul to the abode of Lord Shiva. He learnt then for the first time of the great merit he had earned by unconscious worship of Lord Shiva during the night of Shivaratri. The messengers told him that there had been a Lingam (a symbol for the worship of Shiva) at the bottom of the tree. The leaves he dropped had fallen on the Lingam, in imitation of its ritual worship. The water from his leaky canteen had washed the Lingam (also a ritual action), and he had fasted all day and all night. Thus, he unconsciously had worshipped the Lord. At the conclusion of the tale the King said that he had lived in the abode of the Lord and enjoyed divine bliss for a long time before being reborn as Chitrabhanu. This story is narrated in the Garuda Purana.
‘Sivaratri’ means ‘night of Lord Siva’. The important features of this religious function are rigid fasting for twentyfour hours and sleepless vigil during the night. Every true devotee of Lord Siva spends the night of Sivaratri in deep meditation, keeps vigil and observes fast.
The worship of Lord Siva consists in offering flowers, Bilva leaves and other gifts on the Linga which is a symbol of Lord Siva, and bathing it with milk, honey, butter, ghee, rose-water, etc.
When creation had been completed, Siva and Parvati had been living on the top of Kailas. Parvati asked: “O venerable Lord, which of the many rituals observed in Thy honour doth please Thee most?” Lord Siva replied: “The thirteenth night of the new moon, Krishna Paksha, in the month of Phalguna (February–March) is known as Sivaratri, My most favourable Tithi. My devotee gives Me greater happiness by mere fasting than by ceremonial baths, and offerings of flowers, sweets, incense, etc.
Just hear, My Beloved, of an episode which will give you an idea of the glory and power of this ritual, said Lord Shiva to Parvati.
“Once upon a time, there lived in the town of Varanasi a hunter. He was returning from the forest one evening with the game birds he had killed. He felt tired and sat at the foot of a tree to take some rest. He was overpowered by sleep. When he woke up, it was all thick darkness of night. It was the night of Sivaratri but he did not know it, He climbed up the tree, tied his bundle of dead birds to a branch and sat up waiting for the dawn. The tree happened to be My favourite, the Bilva.
“There was a Linga under that tree. He plucked a few leaves dropped them down. The night-dew trickled down from his body. I was highly pleased with involuntary little gifts of the hunter. The day dawned and the hunter returned to his house.
“In course of time, the hunter fell ill and gave up his last breath. The messengers of Yama(Hinduism) arrived at his bedside to carry his soul to Yama(Hinduism). My messengers also went to the spot to take him to My abode. There was a severe fight between Yama’s messengers and My messengers. The former were easily defeated. They reported the matter to their Lord. He presented himself in person at the portals of My abode. Nandi gave him an idea of the sanctity of Sivaratri and the love which I had for the hunter. Yama surrendered the hunter to Me and returned to his abode. Thereafter, Yama has pledged not to touch my devotees without my consent.
“The hunter was able to enter My abode and ward off death by simple fasting and offering of a few Bilva leaves, however involuntary it might be because it was the night of Sivaratri. Such is the solemnity and sacredness associated with the night”.
Parvati was deeply impressed by the speech of Lord Siva on the sanctity and glory of the ritual. She repeated it to Her friends who in their turn passed it on to the ruling princes on earth. Thus was the sanctity of Sivaratri broadcast all over the world. <http://www.dlshq.org/download/lordsiva.htm>