What is Ganesh Chaturthi and why is it celebrated?

The mythological basis of the celebration is found in the Puranas. One day Parvati was taking bath. She wanted someone to watch outside. She got the idea of making an idol out of the dirt that came from her body. She placed the idol outside. Then her husband Shankar came. The idol dutifully did not allow him to enter. Shankar was furious. In an instant he severed the head of the idol. Parvati finished her bath and came out. She explained to her husband the reason for placing the idol before the door. It was an idol of her own making. In fact the idol was her son ("manas putra"), and consequently also Shankar's son. Shankar felt sorry for what he had done. His servant Gan was standing nearby. Shankar ordered him to go and bring the head of the first living being he would meet. The servant saw an elephant, and he at once cut his head and took it to Shankar. Shankar joined the elephant's head to the body of Parvati's son. "Gaj' means elephant and "anan" means head, so Parvati's son became known as "Gajanan". Shankar made him the deity of his armies and thus his name became Ganesh ("Gan" = army, "Ish" = god) or Ganpati which means "The Lord of Hosts" .