The great importanace of the day is that Shiva got married to Parvathi on this day...
Another legend is that in the course of Armritha Madhanam, the serpent Vasuki spewed out Halahalam or Kaalakoodam.. a poison capable of destroying the whole universe, and there was no one to save us.. The merciful Lord simply took the poison in his hands and put it into him mouth.. Matha Parvathi, realizing the fact that the Lord Carried the whole universe in His abdomen, caught hole of his neck so that the poison did not drip in.. Lord Vishnu ensured that the poison did not come out by closing the Lord's Mouth.. So the poison stuck to the Lords Neck and he became Neelagreeva..
The poison is said to have scorching effect and so Dhaara of cool water and other substances like milk, Ghee, Fruit Paste, Sandalpaste etc are offered throughout all Yaamams to the lord to the chanting of Sree Rudram..
Then to drive home the fact that Shiva is very much pleased if someone does some services to him on this day.. there is the hunters story..
A hunter wandered around a forest all through the day without much success in catching any birds or animals land was extremely hungry and desperate.. Night fell, and he did not have the strength to return to his abode.. So, with a lot of difficulty he climbed a tree expecting to be perched there for the nights sleep.. However, the impossible hunger did not permit him to sleep.. Sitting there atop the tree and ruing his fate, he was plucking the leaves from the branches of the tree and throwing them down carelessly throughout the night.. And the tree happened to be a sacred tree of Bilvam and a Shivalingam had formed underneath.. The leaves thrown down formed night long archana with bilvam to Shiva.. And the hunters cold sweat and tears flew down as the cooling abhishekam for the Lord..
The Lord was extremely pleased..
The hunter went his way not knowing the wealth of divine grace he had earned..
When the hunters last day arrived Yama sent his messengers to bring the fellow to his world, but Lord Siva, sent his servants to the place to carry the fellow to the Holy Sivaloka, because he has, though unintentionally, offered Bilvaarchana and Abhishekam to the Lord on the Sivaratri day, keeping awake all through the night..