Shivaratri is great festival of convergence of Shiva and Shakti. Chaturdashi Tithi during Krishna Paksha in month of Magha is known as Maha Shivaratri according to South Indian calendar. However according to North Indian calendar Masik Shivaratri in month of Phalguna is known as Maha Shivaratri. In both calendars it is naming convention of lunar month which differs. However both, North Indians and South Indians, celebrate Maha Shivaratri on same day.
According to one story from Puranas, during the samudra manthan, a pot of poison emerged from the ocean. This terrified the Gods and demons as the poison was capable of destroying the entire world, and they ran to Shiva for help. To protect the world from its evil effects, Shiva drank the deathly poison but held it in his throat instead of swallowing it. This made his throat turn blue, and he was given the name Neelakantha, the blue-throated one. Shivaratri is the celebration of this event by which Shiva saved the world.
According to another legend in the Shiva Purana, once the other two of the triads of Hindu Gods, Brahma and Vishnu, were fighting over who was the superior of the two. Horrified at the intensity of the battle, the other gods asked Shiva to intervene. To make them realize the futility of their fight, Shiva assumed the form of a huge column of fire in between Brahma and Vishnu. Awestruck by its magnitude, they decided to find one end each to establish supremacy over the other. Brahma assumed the form of a swan and went upwards and Vishnu as Varaha went into the earth. But light has no limit and though they searched for thousands of miles, neither could find the end. On his journey upwards, Brahma came across a Ketaki flower wafting down slowly. When asked where she had come from, the Ketaki replied that she had been placed at the top of the fiery column as an offering. Unable to find the uppermost limit, Brahma decided to end his search and take the flower as a witness. At this, the angry Shiva revealed his true form. He punished Brahma for telling a lie, and cursed him that no one would ever pray to him. The Ketaki flower too was banned from being used as an offering for any worship, as she had testified falsely. Since it was on the 14th day in the dark half of the month of Phalguna that Shiva first manifested himself in the form of a Linga, the day is especially auspicious and is celebrated as Mahashivaratri. Worshipping Shiva on this day is believed to bestow one with happiness and prosperity.
At Maha Shivratri, People keep fast of whole day and night and all the Shiv temples e.g. Kashi Vishwanath temple gets congregated by the young and old devotees from the very early morning. They come to the temple to perform the puja of traditional Shivalingam and hope to get what they have prayed to the god. They take bath in the holy water of the Ganga (Symbol of the purity) early in the morning before sunrise and wear a clean clothe after the sacred bath.