How the date is fixed
Anant Chaturdashi falls on the fourteenth day of the bright half of the lunar month Bhadrapada (Shukla Chaturdashi). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Anant Chaturdashi is the fourteenth tithi of the bright half of Bhadrapada, in September, ten days after Ganesh Chaturthi. The day is the Ananta vrata of the Mahabharata, in which Vishnu as Ananta is worshipped and a fourteen-knotted thread tied on the arm. The tithi at sunrise is taken, and the puja is done in the forenoon. It is also the day of Ganesha immersion in Maharashtra and the last day of the Jain Paryushana in the Digambara tradition.
How it is observed
Worship of Ananta with the fourteen-knot thread, and processions carrying Ganesha images to the water for immersion.