Kalashtami falls on the eighth day of the fortnight during the Krishna Paksha (dark phase) of every month and in the month of Sawan, Kalaashtami is on 24th July. On this day, the Kaal Bhairav form of Lord Shiva is worshipped and devotees fast on this day. It is said that Kalashtami of Margshirsh month is considered to be the most prominent Kalaashtami and this Ashtami is celebrated as Kaal Bhairav Jayanti. At the same time, it is believed that on this day Lord Shiva incarnate as Bhairav and Kalbhairav Jayanti is also known as Bhairav Ashtami.
According to the story of Shri Kalaashtami Vrat Katha- Bhairavatami or Kalaashtami, at one time there was a dispute between Shri Hari Vishnu and Brahma over who is the best of them. The controversy escalated to such a level that Lord Shiva had to organize a meeting to resolve it. It was attended by the knowledgeable, the sage-muni, the Siddha saint, etc. Lord Vishnu accepts a decision taken in the House, but Brahma Ji is not satisfied. They seem to insult Mahadev.
Shanti Chitta Shiva could not bear this humiliation and when humiliated by Brahma, he took the form of Raudra. Lord Shankar began to become a Holocaust and the three people were frightened by his rosy appearance. Lord Bhairav appeared in the same Rudra form of Lord Shiva. He rode on Schwan, had a penalty in his hand. He was called a "criminal" because of the penalty in his hand. Bhairav Ji's form was extremely fierce. He cut off the fifth head of The Brahm Dev, then Brahma Dev remembered his mistake. Then the dispute between Brahma Dev and Vishnu Dev ended and he acquired knowledge, destroying his arrogance and arrogance.