One day when a disciple of Lord Ganesha offered him sweets, Ganesha accepted the treats and spent the rest of the day hoarding them. That night, when it was time to go home, he carried all the sweets along, but when he tripped on a stone and skipped away, all the sweets scattered onto the ground. As he picked up the sweets in embarrassment, Lord Ganesha saw that the Moon laughing at him.
Lord Ganesha angrily cursed the moon for laughing, and for being full of pride to him. The Moon quickly rushed to apologize. But lord ganesha out of anger said that from that day forward, the moon would no longer be full all the time, but would disappear and slowly reappear over 15 days time.
Lord Ganesha also said that since the moon had made fun of him on Chaturthi , so anyone who watches at the moon on that date would face troubles & false accusations. To avoid this curse for anyone who accidentally glances at the moon on Ganesh Charturthi, he or she must listen to the story of the Syamantaka Mani, as listed in puranas.