Seventh day Prayers for Navratri is dedicated to Goddess Kalaratri. Navratri Day 7 Mantra in Hindi is given below. It is chanted on March 26, 2015 during Chaitra Navratri. There are three mantras. First is Mool Mantra for seventh day of Navratri. Second is Dhyan Shloka dedicated to Goddess Kalaratri. Third is a Stotra. You can chant also prayers like Lalita Sahasranama on the day.
There is no need to chant all mantras. Most devotees chant just the mool mantra.