Krishna Ashtami is the twenty-third tithi overall, the eighth day of the dark fortnight, falling near the last-quarter Moon when the disc is again about half lit. It shares the character of the eighth tithi, associated with Rudra and with the Goddess, and it is a Jaya tithi. In the Vaishnava tradition Krishna is widely held to have been born on this waning-side eighth tithi in the month of Bhadrapada, the festival of Krishna Janmashtami, one of the most important of the year. In some Shaiva reckonings the dark eighth tithi, Kalashtami, is kept for Bhairava. The panchanga treats Ashtami with some caution for ordinary matters while honouring its major festivals. These belong to the calendar and devotional tradition.
Kalashtami's dedication to Bhairava, Shiva's fierce form, situates it within a distinct current of Tantric and Shaiva practice separate from the Vaishnava weight the eighth tithi carries through Janmashtami in the bright fortnight, and the two festivals, one to a fierce guardian deity and one to a gentle, playful incarnation of Vishnu, sit on opposite sides of the same month without direct connection beyond sharing the number eight. Krishna Janmashtami's calculation, like several major festival tithis, depends on which tithi prevails at a specified hour, in this case midnight, rather than at sunrise, and disagreement between regional panchangas about the correct reference moment is a recurring, publicly debated feature of the festival's annual dating.