Mahalay Shraddha is performed on the tithi of the death of the father of the person doing Shraddha during Pitrupaksha. The details of preparation required for the ritual and the method of the ritual are mentioned in this article.
If the idol of Shri Ganesh is sculpted as per the science behind the idol then the pure spiritual particles of Shri Ganesh get attracted towards the idol to a greater extent and those worshipping it are benefited.
By immersing the idol in flowing water the Ganesh Principle in the idol also flows and spreads far and wide. With the evaporation of the water charged with the Ganesh Principle, the entire environment also becomes sattvik.