The Origin of the festival lies in the Holy Hindu scriptures which tell the story of Lord Ganesha.
Lord Ganesha (or Ganapati) (the names mean "Lord [isha] or [pati] of Shiva's hosts [gana]") was created by goddess Parvati, consort of Lord Shiva.
According to the legend, Lord Shiva, the Hindu God of destruction, was away at a war. His wife Pavarti, who was a goddess in her own right, wished to bathe and having no-one to guard the door to her apartments, conceived of the idea of creating a son who could guard her. Parvati created Ganesha out of the sandalwood paste that she used for her bath and breathed life into the figure. She then set him to stand guard at her door and instructed him not to let anyone enter. In the meantime, however, Lord Shiva returned from the battle but as Ganesha did not know him, he stopped Shiva from entering Parvati's chamber. Shiva,enraged by Ganesh’s impudence, drew his sword and cut off Ganesha's head. Pavarti emerged to find Ganesha decapitated and flew into a rage. She took on the form of the Goddess Kali and threatened destruction to the three worlds of Heaven, Earth & sub terrain. Seeing her in this mood, the other Gods were truly afraid and Shiva, in an attempt to pacify Parvati, sent out his ganas, or hordes, to bring the head of the first living being with his head towards the north (the auspicious direction associated with wisdom). The first living thing they came across was an elephant. So they brought the head of this elephnat and Shiva placed it on the trunk of Parvati's son and breathed life into him. Parvati was overjoyed and embraced her son, the elephant-headed boy whom Shiva named Ganesha.
2-3 months prior to the day of Ganesh Chaturthi, Life-like clay models of Lord Ganesha are made for sold by specially skilled artisans. They are beautifully decorated & depict Lord Ganesh in various poses. The size of these idols may vary from 3/4th of an inch to over 25 feet.