Navaratri is considered as a festival that fulfills all the wishes of the devotees. In which devotees worships Mata Rani to please her and observe the fast of Navaratri. Mata Rani is pleased to give Shraddalu Gana their desired boon as in people wanting child, money and good education gets it.
By the way, every day of Navratri has special significance, but the eighteenth and nineteenth (Navami) Navratri are termed as very special days, due to which the fast is passed on the Ashtami-Navami date of Sharadiya Navratri.
It is believed that worshiping a girl on Ashtami has special significance. Today we will know why Navami is after Ashtami.
According to mythological belief, there used to be a demon named Mahishasura in the demon world, who did austerities for a long time to please Lord Brahma. Pleased with this, Lord Brahma said to Mahishasura, "Ask what boon do you want to ask for?"
Mahishasura asked for his boon here, that he cannot be killed by neither from the hands of any deity nor from the hands of any man, then Lord Brahma gave him his desired boon and told him that he could only be killed by a woman.
On receiving the boon, Mahishasura became so arrogant that he considered himself God and deposited his suzerainty everywhere and invaded everywhere and established his monarchy and then in his pride, he also attacked the Dev Lok, in which the Dev people In the Trahimaan - the voices of the Trahimans were echoed everywhere. The throne of Lord Indra passed into the hands of Mahishasura to get a huge battle between the gods and asuras in which Lord Shiva and Vishnu also joined, but due to boon no one was able to kill Mahishasura and was unable to win, then Lord Vishnu created an extremely beautiful Apsara named Durga which was a form of Shakti with his splendor.
On top of which Mahishasura became fascinated and proposed marriage to Shakti Rupa Durga, on which Shakti Rupa put a condition that if you want to marry me, you must first fight me.
Mahishasura accepts the proposal of Shakti Swaroop and there is a war between Mata Swaroopa Durga and Mahishasura on the entire Navami day in which Mata Rani killed asur Mahishasura.
Since then Navami is considered to be special in Navratri.
On the other hand, it is also mentioned in the Mahabharata written by Ved Vyas. It is said that Lord Krishna once told Dharmacharya, Yudhishthira and Arjuna the importance of Ashtami & Navami, due to which Yudhishthira worshiped mother power.
Navami's is related with God Ram as well-
Mata Rani is worshiped on the day of Navratri, in which we also know Navami by the name of Ram Navami. According to the mythological belief, Shri Ram Bhagwan was born on the Navami of the Shukla Paksha of Chaitra month, which we also know the name of Ram Navami. This signifies it as also the birthday of god Ram. On the other hand, the Dashami of the Shukla Paksha of Ashwin month is celebrated as the victory of good over evil.