On the fourth day of the first fortnight in the month of Bhaadrapada (on Ganesha’s birthday), lord Ganesha was returning home after having a sumptuous feast at a devotee’s house. He was riding on his divine vehicle, a tiny mouse. At the sight of a snake crossing across the path, the mouse got scared and panicked, causing well-fed Ganesha to fall. The chaos got intensified as his stomach burst open. Ganesha killed the snake and tied it around his stomach to prevent it from further rupturing. Watching all these incidences, the moon began to laugh loudly. The moon sarcastically laughed because of his arrogance, flamboyance and ignorance about the great light source.
The humiliated Ganesha cursed the moon saying that no light would ever fall on him. The moon disappeared from the sky resulting in chaos on Earth. Gods and humans pleaded Ganesha to take back the curse. Moon also understood his mistake and pleaded pardon from him. Ganesha succumbed to the request and modified the curse saying that anybody who looked at the moon on his birthday (4th day Chaturthi of the Bhadrapada month) would suffer from defamation allegations for the rest of the year. This is the reason why devotees refrain from looking at the moon on this auspicious day. It is believed that the moon endures regular waxing and waning due to the curse.