Atrismruti says -
surye kanyagate kuryaacchraadham yo na gruhashramee
dhanam putrankulam tasya pitrunisvasapeedaya
kanyagate savitari pitaro yanti satsutaan
shunya pretapri sarvaa yavadvrushchikadarsanam
tato vrishchikasampraptou nirashaah pitaro gataaha
punah svabhavanam yanti shaapam datvaa sudaarunam
putram va bhrataram vapi douhitram poutrakam tatha
The pitrupaksha is the period from the krishna pratipada to amavasya when sun is transiting kanya rasi. 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 vrsichika rasi. 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.