In most beliefs Amavasya is not auspicious. It is the day when the evil spirits are stronger. Tantra is mostly performed on an Amavasya day. The evil spirits are free on this day. This is more of the contribution that movies have made. But black magic can be stronger when done in Amavasya. Diwali is considered to be the most inauspicious Amavasya. This is exactly why Gods are worshipped and lights are lit all across the house so that no evil spirit can enter the house. This is also the reason why goddess Kali is worshiped on this day in some parts of the country as Goddess Kali has the power to destroy all the evil.