In her Parvati form she practiced severest possible penance with a desire to have Lord Siva as her divine consort. She took a vow (Narada Pancaratra). In the words of Tulasidasa her firm vow was-
Due to the severe penances her body became black. Lord Siva was pleased in the end. He himself washed her body with the holy water of Ganga. Then her complexion was changed into a shining one like a lightning flash. Since then she was known as Mahagauri.
The worship of Mahagauri is recommended for the eighth day of Navaratra Puja. Her power is unfailing and instantly fruitful. All sins of devotees are washed away with the result of her worship. Even accumulated sins get destroyed and in future no sins or sufferings come to him. He gets purified in all respects and becomes fit for inexhaustible virtues.
The meditation or remembrance or worship of Mahagauri is benedictory for the devotees in every respect. We should always meditate upon her. Divine Siddhis are obtained from her grace. A devotee should always meditate on her lotus-feet single-mindedly. She definitely removes the pains of her devotee. As a result of her worship the afflicted devotees get even impossible done possible. So, everybody should try by all means to get shelter under her feet. The Puranas are replete with the narrations glorifying her. She channelises the thought waves of the devotees towards the path of the virtue and destroys Asat.