Once upon a time Goddess Parvati was taking bath. While bathing she created Ganesha of mud and placed him at the entrance of the house then she instructed him to keep guard that home. She told him not to allow anyone while she was taking bath. After some time when lord Shiva was returning to the home. He was quite thirsty and was stopped by Ganesha at the gate of the house. Then lord Shiva became angry and they both fight with each other. Lord Shiva cut off Ganesha’s head.
When Parvati came to know of this, she was being so sad. To console her, Shiva ordered his servants to cut off and bring to Him the head of any creature that might be sleeping with its head facing in the north. The servants went on their mission and found only an elephant in that position. The sacrifice was thus made and the elephant’s head was brought to Shiva.
Lord Shiva fixed the elephant’s head onto the body of Ganesha. Lord Shiva made his son worthy of worship at the beginning of all undertakings, marriages, expeditions, studies etc. He said that the annual worship of Ganesha should take place on the 4th day of the bright half of Bhadrapada.
Without the Grace of lord Ganesha and His help nothing what so ever can be achieved. No action can be undertaken without His support.