When the moon had to face Parvati's wrath for Ganesha.
Ganesha had stuffed himself at Kubera's house on his birthday and it was difficult for him to even move. His pot-belly made it difficult for him to even walk, so he stumbled and fell on the ground. Legend has it that the moon was watching all of this and he started laughing and made fun of him. This angered Parvati, who threw a curse at the moon, saying that whoever sees the moon will be blamed without no reason and it would turn black and remain invisible. Upon realizing its mistake, the moon pleaded for forgiveness after which, Parvati said that the moon will wax and wane every fifteen days and who ever prays to Lord Ganesha on Ganesha Chaturthi will be survived from uninvited blame.