Mahashivratri or Shiva`s great night falls on the fourteenth night of the New Moon, during the dark half of the month of Phalguna (February - March). It is said that, on the night of Shivratri, Shiva manifested himself for the first time in the form of a lingam. The Lingam is worshipped to remove the sins committed during the year. There is a legend associated with the celebration of this festival, which says that Lord Shiva showed himself to Lord Brahma and Lord Vishnu, in the form of Lingam. When he did, he also announced to the two deities that he was their progenitor and also was the Creator, the Preserver and the Destroyer. He said that Brahma was his right side and Vishnu his left and they all collectively existed in Shiva. It was here, that Shiva lay down that he must be worshipped in his phallic form and not in his anthropomorphic form.
The essential religious activities during the festival are vrata (fast) for the whole day, Yajna
(sacrifice), keeping a strict night vigil and worshipping the Lingam with flowers, leaves of Bel tree and food with recitation of mantras (hymns). The Bel is sacred to Shiva and offerings of its leaves particularly please the deity. Every orthodox Hindu keeps awake by engaging in pious activities. Vrata (fast) is observed, mainly to achieve physical and mental self-control. It is a way to show gratitude and to praise the Supreme God, Shiva. The fast is also kept in the hope of a special boon. Fairs held during the festival are also considered honorable.
For a Yajna, one should construct a three-tiered platform, which represents the three levels of the universe- swargaloka, antariksha-loka and bhuloka (heaven, space and earth). Then eleven kalashas or kumbhas (earthen pitchers), filled with water, are placed on a raised platform. These represent the eleven forms, in which Rudra-Shiva manifests himself to the world. The lowest of the three tiers of the platform represents the earth and is left vacant. Ten kalashas are placed on the middle platform to represent the middle region of the universe i.e., antarikshaloka.
Two kalashas each are placed on the eastern and western sides; three each on the northern and southern sides. These are filled with water and perfumed with sandalwood and rosewater. On the highest level of the platform is placed the largest kalash, which represents the essence of Surya as well as the ethereal regions. The major portion of the offerings and oblations go to this level, as Shiva in a way, is also Surya. Each kalash is wrapped in string netting and decorated with the bilva and mango leaves. A narial (coconut) is put on the mouth of the kalash that is draped with a single strip of cloth. The coconut represents the head of Shiva, the outer fibrous covering the matted hair of Shiva the ascetic, and the three black spots on it, his three eyes.
A silver or golden plate is placed inside the largest Kalash and on it a lingam is placed. The lingam is bathed with the five sacred gifts of the cow called panchagavya - milk, curd, clarified butter, honey and sugar. After this, sacred mantras (hymns) are recited and til (sesame seeds), grains of rice, boiled rice and clarified butter are offered. Sacrificial flowers like dhatura and jati, traditionally considered sacred to Shiva, are also offered. Eleven Brahmin priests sit in a circle round the platform and continuously chant mantras. During the chanting of mantras, the priests carry a rosary made of rudraksha beads. By such an invocation, Shiva takes residence in the eleven vessels on the altar.