Kalaashtami is the day on which Kala Bhiarav, a fearsome form of Lord Shiva is worshipped. It occurs on the eighth day after the full moon in the Hindu month of Kartik. Kala Bhairav is worshipped on Kalaashtami since Lord Shiva created him on this day and hence is even called the birthday of Kala Bhairav.
According to Hindu Mythology, Kala Bhairav was created by Lord Shiva due to his anger towards God Brahma. In Hinduism, there are three major gods known as Brahma, Vishnu and Shiva and once all of them had an argument over who is more supreme, a comment by Brahma enraged Shiva so much so that he created Kala Bhairav to chop 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 said to be one of the most fearsome warriors and guardians. Devotees show their dedication by praying to Kala Bhairav from midnight till dawn. In the morning, people give food to dogs as a way of pleasing Kala Bhairav since his riding vessel was a dog.
Team AskGanesha wishes you all a very Happy Kala Ashtami.
12th April: Kala Ashtami by Acharya Dharam Shastri