Kalashtami, which is also known as Kala Ashtami, is experiential every month during Ashtami Tithi of Krishna Paksha. Devotees of Lord Bhairav keep fast and worship Him on all Kalashtami days in the year.
The most important Kalashtami, which is known as Kalabhairav Jayanti, falls in the month of Margashirsha according to North Indian, Purnima to Purnima, lunar month calendar while Kalabhairav Jayanti falls in the month of Kartik in South Indian, Amavasya to Amavasya, lunar month calendar.
However both calendars observe Kalabhairav Jayanti on the same day. It is believed that Lord Shiva was appeared in form of Bhairav on the same day.
Kalabhairav Jayanti is also known as Bhairav Ashtami.