- What is the reason behind this?
- Also, why is visarjana (immersion) only done on certain days e.g., 1.5, 5 or 11 days after the idol has been installed?
Every year Clay statues of Lord Ganesha are immersed into the water bodies or in sea. Gradually the clay dissolves into water. Ganesha's image disappears. This ritual is designed by our ancestors to draw attention to the 'ephemeral nature of life and worldly things'. Showing that nothing in this mortal world lasts forever. It celebrates the fact of life of Creation(Idol construction),Perpetuation and finally Visarjan or immersion(Destruction) in water bodies with fun and fair, as celebration pointing to the fact and Showing that death is imperative/essential for creation of new that's why people say
"गणपति बब्बा मोरिया अगले वरस तू जल्दी आ !".
Source : wiki.answers.com
However, many people who keeps a Ganesh in their homes choose to perform the immersion much before this. There are no hard and fast rules as to when it's done. Rather it's up to personal preference.
Source : Ganesh Visarjan
There is more to ganesha visarjan than just creation or destruction principle of life. Actually when we worship a sthool object (material object) it actually becomes one of the Jagrit (active) form of god. When that material object's purpose or our purpose has been completed we have to destroy it , but before destroying it we have to make sure there are no godly elements remain in it and it becomes a inactive god object again (supta form) like every other material object.
If we directly put it into water or dump the object, it will be disrespect of the energy of the supreme god. So to avoid this we pray to god to leave that object before we destroy it (by visarjan mantras depicted in texts) so that after sprinkling the abhimantrit water (enchanted water) on the idol the energy gets released and goes to its udgam sthan (the origin) and thus a cycle is complete then we do the Visarjan.
answer to #8 question:
After shiva cut off ganesha's original head with his trisul the head rested on shiva's trisul flew to brahmlok and came back in the form of a coconut. This shiva had to sacrifice his son's head to fulfill Parvate wish (maang).
Parvati once asked lord shiva that you have a lot of sick devotees and to do your nirajal vrat is impossible for them. If they partake milk or even a fruit their vrat will be spolit. So mata parvati asked if Shiva could produce a fruit with little water in it for his sick devotees so that they can partake the fruit inside the coconut and drink the water to maintain their vrat. please bare in mind you are allowed to have only one coconut during the entire vrat even if you are sick