The date of Ganesh Chaturthi falls on the fourth day after the new moon in the Hindu month of Bhadrapada. This is August or September each year. It's commonly celebrated for the following 11 days, with the biggest spectacle taking place on the last day called Ananta Chaturdasi.
In 2011, Ganesh Chaturthi is on September 1 (Ananta Chaturdasi on September 12).