Lord Ganesha or Ganpati Bappa as HE is fondly called is brought home with huge celebrations on Ganesh Chaturthi. People set up beautiful pious places to make HIM feel at home . Lord Ganesha is kept at home with all ritualistic worship, fed with sweets ( Modaks being HIS favourites as per Hindu Mythology ) , decorated with flowers , with proper aarti and pooja, you are supposed to look after Ganpatiji and take proper care of HIM, people come to your home to see HIM. And then, on the second day/fifth day , a huge pooja is done again to thank HIM for blessing the house and ask HIM to come again next year. HE leaves the house with great celebrations again , HIS pooja is done again at water and then, the family says bye to Lord Ganesha as HIS idol goes into water.
Lord Ganesha is is the God of success and HE is looked after like family during those ten days. Devotees are extremely sentimental on the day of Visarjan ( the going of Lord Ganesha into water).