Ganesha's birth, as the Shiva Purana tells it. As Parvati prepared to bathe, she shaped a boy from the paste and dirt rubbed off her own body, gave him life and set him at the door: no one was to enter. Shiva came home, and the boy, who had never seen him, barred his way. Words turned to battle; the ganas could not move the boy, and at last Shiva cut off his head.
Parvati's grief and fury shook the worlds. To calm her, Shiva sent the gods north to bring the head of the first living being they met; they came back with an elephant's. Shiva set it on the boy's body and brought him back to life, made him lord of his ganas, Ganapati, and decreed that he would be worshipped before every other god, at the start of every rite. Tradition keeps his birthday on Bhadrapada Shukla Chaturthi, at midday.
Why the moon is not seen. On his birthday, the story goes on, Ganesha was riding home on his mouse, heavy with the modaks of his feast, when the mouse stumbled and he fell. The Moon, proud of his own beauty, laughed. Ganesha cursed him: whoever looked at him would be blamed for something they had not done. When the gods pleaded, Ganesha softened the curse to this one day: whoever sees the moon on Bhadrapada Shukla Chaturthi faces a false accusation, a mithya dosh. That is why the day is also called Kalank Chaturthi, the Chaturthi of blame. The tellings differ in detail; the curse is the same in all of them.
The Syamantaka jewel. Even Krishna did not escape it. Satrajit of Dwarka had the Syamantaka jewel from the Sun. His brother Prasena wore it hunting; a lion killed him and took it, and the bear-king Jambavan killed the lion and carried the jewel to his cave. Dwarka whispered that Krishna had killed Prasena for it, and Krishna, the tellers add, had glimpsed the moon on this Chaturthi.
Krishna followed the trail into Jambavan's cave, fought him for many days, and won both the jewel and Jambavan's daughter Jambavati as his wife. He returned the jewel to Satrajit before all of Dwarka, the false charge fell away, and Satrajit gave him his daughter Satyabhama in marriage. Hearing this story, tradition says, lifts the blame of seeing the moon by mistake, which is why it is read on this Chaturthi.
Why Ganesha is worshipped first, as a popular telling has it. Shiva and Parvati once set their sons a race around the world. Kartikeya flew off on his peacock; Ganesha, on his small mouse, simply walked around his parents and bowed: they were his world. The prize was his, and with it, the telling says, the place he keeps to this day, first in every worship; in the Shiva Purana the prize is the right to marry first.
Ganesh Chaturthi celebrates the birth of Ganesha, Vighnaharta, the remover of obstacles and the lord of beginnings, worshipped first in every puja, wedding and new venture. Families welcome him home as a beloved guest, care for him for one and a half to ten days, and send him into the water with the cry: come back soon next year. The clay returning to the water is part of the meaning: the form goes, the presence stays.
The festival was kept at home for centuries, and grandly at the Peshwas' court in Pune in the eighteenth century. In the 1890s, after the first public Ganeshas appeared in Pune, Bal Gangadhar Tilak championed it as a public, sarvajanik festival in Pune and Mumbai, so that people of every caste and class could gather around one Ganesha, and the pandals that fill cities today grew from that.
Ganesha has a Chaturthi every fortnight: Vinayaka Chaturthi in each bright half and Sankashti Chaturthi in each dark half. Magha Shukla Chaturthi is kept as Ganesh Jayanti, and Magha Krishna Chaturthi as Sakat Chauth in the north. Bhadrapada Shukla Chaturthi, this day, is the great one.