Kalabhairava Jayanti — Shiva Purana 3.9.63-64

This pair of verses is the textual basis of Kālabhairava Jayantī, also called Bhairava Aṣṭamī — the eighth of the waning fortnight of Mārgaśīrṣa, which falls in November or December. The monthly Kālāṣṭamī, on the eighth of every waning fortnight, is the same observance at a smaller interval. The second verse names the two things done: upoṣya, having fasted, and jāgaraṃ kurvan, keeping the vigil. Both are night practices, which is consistent with everything else about this deity — the eighth of the dark fortnight is a night with little moon. Verse 65 immediately extends it: anyatrāpi, 'elsewhere too', someone who keeps the vow with devotion has the same standing. The Purana is explicit that presence at Kashi is not required, which is what allowed the observance to travel. mahāpāpaiḥ pramucyate is the text's own statement about the observance, given here as what the verse says. Nothing on this page restates it as an outcome, and the vrata is described rather than prescribed.