Why people don't look at the Moon on that day?

There is very popular legend regarding this context. Lord Ganesha was very fond of sweet pudding or Ladoos. On one of His birthdays, He was going around house to house accepting the offerings of sweet puddings.

Having eaten a good number of these, He set out moving on his mouse at night. Suddenly the mouse stumbled as it had seen a snake and became frightened with the result of that Lord Ganesha fell down.

His stomach burst open and all the sweet puddings came out. But Lord Ganesha stuffed them back into his stomach and, caught hold of the snake and tied it around his belly.

Seeing all this, the moon in the sky had a hearty laugh. This unseemly behaviour of the moon annoyed Lord Ganesha immensely and so he pulled out one of his tusks and hurled it against the moon, and cursed that no one should look at the moon on the Ganesh Chaturthi day. If anyone does, he will surely earn bad luck.