It was customary to worship Durga with 100 neel kamals (blue lotuses). Rama could gather only 99 of them; he offered one of his eyes as a substitute of the 100th lotus. Pleased with his devotion Durga blessed him. The battle started on Shaptami and Ravana was killed at the Shandhikshan (in between) Ashtami and Navami; he was cremated on Dashami. Therefore the four days of the puja that we celebrate ends in the triumph of the good over evil.