Importance of sraadha during pitrupaksha or mahalayapaksha
Atrismruti says -
suryekanyagatekuryaacchraadham yo nagruhashramee
The pitrupaksha is the period from the krishnapratipada to amavasya when sun is transiting kanyarasi. At the commencement of this period the pitrus leave pitruloka and reach their descendants. The pitruloka becomes empty then. They remain on earth till sun enters vrsichikarasi. They come with great expectation. If they are not propitiated by the performance of sraadha in the pitrupaksha, they get greatly disappointed and their sigh of grief will destroy the wealth, children and kula of the descendants. They return after issuing terrible curse upon the non performing relatives such as sons, brothers and grandchildren.