Navratri is divided into sets of three days to adore 3 different aspects of the supreme goddess or goddesses.
first 3 days:
the goddess is separated a spiritual force called durga also known as kali in order to destroy all our evil and grant boons.
second 3 days:
the mother is adored as a giver of spiritual wealth, lakshmi, who is considered to have the power of bestowing on her devotees inexhaustible wealth, as she is the goddess of wealth.
final 3 days:
the final set of three days is spent in worshiping the goddess of wisdom, saraswati. in order to have all-round success in life, believers seek the blessings of all three aspects of the divine femininity, hence the nine nights of worship.
happy navratri!!