Ganesha’s birthday, celebrated on the foruth (chatur) day after new moon in the month of Bhadrapada (August/September).
On Ganesha Chaturti one shouldn’t look at the moon because he once laughed at Ganesha. A devotee offered him lots of sweets, he couldn’t eat them all and stuffed the rest into his pockets. On the way home his mount, the rat Mushika, stumbled, Ganesha fell down, the sweets rolled on the street. The moon laughed out loud.
Ganesha said: You laughed at me at Chaturti. Anyone who looks at you on this day will have big problems in future.