Significance of Navaratri
Navaratri represents a celebration of the Goddess Amba(the Power). In gujarat(India) we do Aarti of Mother Amba all 9 days and play Garaba.
Navaratri or Navadurga Parva happens to be the most auspicious and unique period of devotional sadhanas and worship of Shakti (the sublime, ultimate, absolute creative energy) of the Divine conceptualized as the Mother Goddess-Durga, whose worship dates back to prehistoric times before the dawn of the Vedic age.
Significance of Nav - durga incarnations
Name - worship day on navratri - Planet Govern
Shailputri - Day 1 - Moon
After the self-immolation as Goddess Sati, Goddess Parvati took birth as the daughter of Lord Himalaya. In Sanskrit Shail means the mountain and due to which Goddess was known as Shailputri, the daughter of the mountain.
Brahmacharini - Day 2 - Mangal
Goddess Parvati took birth at the home of Daksha Prajapati. In this form the Goddess Parvati was a great Sati and her unmarried form is worshipped as Goddess Brahmacharini.
Goddess Brahmacharini did severe penance to get Lord Shiva as her husband. She performed hard austerity and due to which She was called Brahmacharini.
Chandraghanta - Day 3 - Shukra
Goddess Chandraghanta is the married form the Goddess Parvati. After getting married to Lord Shiva Goddess Mahagauri started adorning her forehead with half Chandra and due to which Goddess Parvati was known as Goddess Chandraghanta.
Kushmanda - Day 4 - Aun
Goddess Parvati started living inside the center of the Sun so that He can liberate energy to the universe. Since then Goddess is known as Kushmanda. Kushmanda is the Goddess who has the power and capability to live inside the Sun. The glow and radiance of her body is as luminous as that of the Sun.
Skandamata - Day 5 - Budha
When Goddess Parvati became the mother of Lord Skanda (also known as Lord Kartikeya), Mata Parvati was known as Goddess Skandamata.
Katyayani - Day 6 - Brihaspati(Guru)
To destroy demon Mahishasura, Goddess Parvati took the form of Goddess Katyayani. It was the most violent form of Goddess Parvati. In this form Goddess Parvati is also known as Warrior Goddess.
Kalratri - Day 7 - Shani
When the Goddess Parvati removed outer golden skin to kill demons named Shumbha and Nishumbha, She was known as Goddess Kalaratri. Kalaratri is the fiercest and the most ferocious form of Goddess Parvati.
Mahagauri - Day 8 - Rahu
According to Hindu mythologies, the Goddess Shailputri at the age of sixteen was extremely beautiful and was blessed with fair complexion. Due to her extreme fair complexion she was known as Goddess Mahagauri.
Siddhidatri - Day 9 - Ketu
In the beginning of the universe Lord Rudra worshipped Adi-Parashakti for creation. It is believed that Goddess Adi-Parashakti had no form. The supreme Goddess of Power, Adi-Parashakti, appeared in the form of Siddhidatri from the left half of Lord Shiva.
Reference : Markandey Purana (In Durga Saptshati, the Markandeya Purana depicts nine embodiments of the Goddess)
Also See: Nav Durga on Wikipedia and Devi Mahatmya