One Ganesh Chaturthi night, after enjoying the many modaks his devotees had offered, Ganesha was riding home on his mouse. On the way the mouse stumbled and Ganesha fell to the ground.
Watching this from the sky, the Moon god, proud of his own beauty, burst out laughing. Ganesha cursed the Moon: whoever looks at you will be falsely accused.
When the Moon begged forgiveness, Ganesha softened the curse: only seeing the moon on the night of Bhadarva Sud Choth (Ganesh Chaturthi) would bring blame. That is why there is a tradition of not looking at the moon that night. According to the story, even Lord Krishna was falsely accused of stealing the Syamantaka jewel because he saw the moon on this day.
The fourth day of the dark fortnight of every month is called Sankashti Chaturthi, when devotees fast and worship Ganesha after the moon rises.