What is the Islamic status of the 3rd, 7th and 40th day customs held for the dead?

Answer

There is no origin in the Sunnah for the 3rd, 7th and 40th day dua/ khatam customs. These customs were innovated many centuries after the Sahaabah. These practices are bid'ah – haraam innovations. Hindus of India too recite their holy book on these occasions.