When there's Ganesha Chaturthi going on, people say that we shouldn't look at the moon on the day of immersion (visarjan) as its counted as a bad omen.
So why did he cursed the moon and why is that, we shouldn't look at the moon?
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There are various version of this story but in all story Moon laughed at Lord Ganesha dn got cursed.
On the 4th day of the Bhadrapad month, Lord Ganesha was returning to his abode on his vehicle, the mouse. He had been fed well by his devotees. Seeing this sight of the huge Lord on a tiny mouse, the moon in the sky began to laugh aloud. He was vain about his splendor and beauty and thus he laughed. Little did he know that all his splendor was only a ‘reflection’ of the Sun God. In order to teach the vain moon a lesson, Lord Ganesha cursed that no light ever fall on him again. Thus, the moon had no light and disappeared from the skies.
The moon repented and sought pardon from Lord Ganesha. All the other beings of the world too pleaded on his behalf to Ganesha and the Lord acceded. However, once a curse is pronounced, it cannot be taken back, even by God. It can only be modified. Thus we are advised to be careful about our speech in everyday life. Lord Ganesha modified the curse and said that the moon would undergo regular waxing and waning to remind him to be humble. He also added that anyone seeing him on the 4th day (Chaturthi) of the Bhadrapada month would face blame and unjust criticism. (Source:hubpages.com)
This is the same story, I listened in Ganesh Chaturthi Katha. This same curse is responsible for Lord Krishna to be blamed for stealing a noble gem.
Lord Ganesha cursed the Moon god because once he made fun of the form of form of Ganesha (which is elephant-headed). The story is given in the Khanda 4 and Chapter 6 of Mudgala Purana. The relevant verses are:
एतस्मिन्नंतरे तत्र चन्द्र: शंकरभालग:।
जहास विवृतैर्दंतैर्गणेश रूपगर्वित:।।
उवाच मोहितोऽत्यंतं प्रणम्य गणनायकम्।
गजवक्त्रयुतं रूपं किं धृतं विकटं प्रभो।
पुनः पुनरुवाचेदं वाक्यं हास्यपरायणः।
ततस्तं कुपितो देवः शशाप शशलाञ्छनम्।।
ये त्वां विलोकयंते वै चन्द्र ते पापिनः सदा।
भवंतु विघ्नसंयुक्ता मद्वाक्यान्नात्र संशय:।।(Mudgala Purana 4.6)
There that Moon which was situated on the head of Shankara started laughing at Ganesha showing his teeth due to pride in his own form. By becoming deluded he spoke to Ganesha as: 'Your form with this elephant head is so funny!' Saying this again and again he laughed. This made the Lord (Ganesha) angry and cursed: 'Those who will see Moon will always be sinner. They will get full of obstacles.' This word of mine shall not undergo false.
After that incident moon god also became powerless and creation also is effected negatively due to Soma tatwa (Moon element) being powerless. All gods and Moon God also pray to Lord Ganesha and after various austerities Lord Ganesh appears before them and tells Gods to ask for boon:
यदि प्रसन्नभावेन वरदोऽसि गजानन।
तदा चंद्र सुनिर्दोषं कुरू नाथ नमोऽस्तु ते।।
स तानुवाच सर्वात्मा भक्त्या संतोषितः प्रभुः।
भाद्रशुक्लचतुर्थ्यां वै न द्रष्टव्यः सुधाकरः।।
हास्यं कृतं मदीयं वै तद् दिने तेन देवपाः।
दोषयुक्तो विधुस्तस्मान्नान्यथा मे वचो भवेत्।।
If you are pleased with us oh Lord Gajanana give us a boon that Moon should be freed from the defect (of your curse), we pray to you, Lord. Becoming pleased with their devotion the Lord who is self of all spoke then: 'Do not see the moon on the day of Bhadra Shukla Chaturthi (4th bright fortnight of Bhadra month). The moon had laughed at me on that very day and hence that defect (curse) will be applicable on that day only and not otherwise.'
So, the curse is now applicable only on the day of Bhadra Shukla Chaturthi (4th bright fortnight of the month of Bhadra). Also as per Ganesha Purana chapter 72 if someone mistakenly sees moon on this day he should do Chintana (contemplation/meditation) on the form and deeds of Lord Ganesha!