Kalashtami or Kala Ashtami is a ritual observed by the Devotees of Lord Bhairav during Ashtami Tithi of Krishna Paksha on every month. They observe fast and worship Lord Bhairav on all Kalashtami days.
Kalabhairav Jayanti, also known as Bhairav Ashtami, is regarded as the most significant Kalashtami, which falls in the month of Margashirsha as per North Indian, Purnima to Purnima, lunar month calendar. Whereas, Kalabhairav Jayanti falls in the month of Kartik according to the South Indian, Amavasya to Amavasya, lunar month calendar. Only the calendar names differ, but both these calendar days of Kalabhairav Jayanti fall on the same day. It is believed that Lord Shiva manifested himself as Bhairav on the same day.
Kalashtami fasting might be observed on Saptami Tithi. As per religious text Vratraj, Kalashtami fasting should be observed on that day when Ashtami Tithi is prevalent at night. Accordingly, while selecting fasting day for Kalashtami, Templesinindiainfo.com ensures that Ashtami should prevail at least for one Ghati after Pradosh. Otherwise the Kalashtami fasting day should be shifted to the previous day when the possibility is more for Ashtami Tithi to prevail at night.