Kalabhairava Ashtami

Kalabhairava Ashtami is observed on the 8th day of the Krishna Paksha (waning phase of the Moon) of the month of Mrigashira/Margasheersha.

 According to ShataRudra Samhita of Shiva Purana,
Parameshvara appeared in form of Bhairava the Krishna Ashtami of Margashirsha month.

कृष्णाष्टम्यां तु मार्गस्य मासस्य परमेश्वरः।
आविर्बभूव सल्लीलो भैरवात्मा सताम्प्रियः॥


The ShataRudra Samhita states:

विश्वंभर्त्तुं समर्थोऽसि भीषणद्भैरवः स्मृतः।
त्वत्तो भेष्यति कालोऽपि ततस्त्वं कालभैरवः॥

You are capable of sustaining the world, “terrifying” – thus, remembered as Bhairava
of You will be terrified the Kaala too, thus will You be eulogized as Kalabhairava.

In accordance with the Shiva Purana, Vrata for Kalabhairava can be observed on Shukla Ashtami of Mrigashira month. Worship of Kalabhairava can be observed on all Krishna Paksha Ashtami tithis or even Tuesdays.