Kala Bhairav was an incarnation of Lord Shiva and Kalaashtami is the day dedicated to his worship and is also his birth anniversary. It is celebrated on the eighth day after the full moon in the Hindu calendar month of Kartik. Kala Bhairav is said to be one of the most powerful and fearsome incarnation of Lord Shiva.
According to old scriptures, once there was an argument between the gods of the Hindu trine, Shiva, Brahma and Vishnu. The argument was regarding who is superior of the three, Brahma during this argument made many comments that enraged Shiva who then created Kala Bhairav using his nail. Kala Bhairav in his anger chopped off one of the five heads of Brahma.
Although texts vary as to why the argument may have happened, the conclusion remains the same, that is, Shiva in his rage created Kala Bhairav who cleaved the head of Brahma.
Kala Bhairav is symbolized in Hinduism as a great protector and fearsome warrior. There are not many temple dedicated to him in India but on this day all his devotees visit his temples and fast in his name. They also feed dogs who were the ride of Kala Bhairav.