Overview
Mahagauri (Sanskrit: महागौरी, Mahāgaurī, meaning 'the great fair-complexioned one') is a form of the Hindu goddess Mahadevi. Within the tradition, she is venerated as a goddess associated with beauty and purity.
She is counted as the eighth of the Navadurga, the nine forms of the goddess Durga who are venerated collectively in the Shakta tradition of Hinduism. Each of these forms is associated with a particular day of the festival of Navaratri.
In keeping with her position among the Navadurga, Mahagauri is worshipped on the eighth day of Navaratri. The observance forms part of the wider devotional practices dedicated to Mahadevi during the festival.
References
Adapted from the English Wikipedia article on Mahagauri.