Every month in Krishna paksha chathurdhasi (fourteenth moonday) is called masa Shiva rathri. The one that comes in the month of "Masi" (mid February to mid March) is called Maha Shiva rathri. This is considered as the most important vrata by the devotees.
There are many incidents told about the greatness of this day. Once a hunter in a jungle after searching throughout the jungle, was quite tired and could not get any animal. In the nightfall a tiger started chasing him. to escape from that he climbed a tree. That was a Bilva tree. The tiger sat under the tree waiting for him to come down. The hunter who sat on a branch of the tree was quite tense and didn't want to sleep. He was plucking the leaves and putting down as he was not able to be idle. Below the tree there was a Shiva lingam. The whole night went on like this. God was pleased with the Upavasa (hunger) and the Pooja the hunter and the tiger did even without knowledge. He is the peak of the grace. He gave the hunter and the tiger "Moksha".