Saptami is the seventh day of the Navratri.
The seventh day is dedicated to worship Goddess Kaalratri.
Goddess Kaalratri is known as the most ferocious form of Goddess Durga.
Goddess Parvati had removed her fair skin to kill the demons Sumbha and Nisumbha, hence she looks fierce.
This Goddess appears wearing tiger skin with a lot of rage in her fiery eyes and her skin has turned black in colour.