Durga Ashtami is part of the Navaratri observance rooted in the Devi Mahatmya and the worship of the goddess in her Mahagauri form.
On the eighth day of the battle, the goddess manifested as Mahagauri, a radiant and compassionate form, continuing the fight against the demon Mahishasura.
Devi Mahatmya; regional Navaratri tradition