Kali Chaudash is observed a day before Diwali in North India and
Gujarat. It is also known as Narak Chaturdashi. Deepavali in South India is celebrated on the day. In 2017, date of Kali Chaudash is October 18. Legend has it that Lord Krishna killed demon Narkasur on the day with the help of Sathyabhama. Special prayers are also offered on the day to Lord Hanuman. Hanuman puja is a major spiritual event on the day for many Hindu families.
People bath before sunrise on the day. Special prayers are offered to Lord Hanuman. Hanuman is believed to remove the fear of evil spirits and assure in auspiciousness on the day.
You can read more about the Legend of Narak Chaturdashi here.