Kalashtami or Kala Ashtami would be the day when Lord Shiva manifested the form of Kaal Bhairav, so this day is an important day for the devotees of Lord Bhairava. This day falls every month according to the Hindu calendar. On this day, devotees of Lord Bhairav keep fast throughout the day and worship them on the day of Kalashtami in the year.
Kalashtami, also known as Kala Ashtami. By the way, the fast of Kalashtami is observed on the day of Bhairav Jayanti. Bhairava is the incarnation of Lord Shiva in the form of anger. It can be said that Bhairava is the manifestation of Lord Shiva's wrath.
In Tantra Sadhana, worship of eight forms of Bhairava is said. These forms are Asitang Bhairav, Rudra Bhairav, Chandra Bhairav, Krodha Bhairava, Manikat Bhairava, Kapali Bhairava, Bhishna Bhairava Sanhar Bhairava.
In the Kalika Purana, Bhairav is described as Lord Ganesha whose vehicle is a dog. The seeker observing this day should chant 'Om Kalabhairavaya Namah' throughout the day. By observing the fast of Kalabhairava, all the wishes of the worshiper are fulfilled. A person doing Bhairav Sadhana gets rid of all sorrows.