Ashtami (8th day of the lunar fortnight) and Chaturdashi (14th day of the lunar fortnight) are said to be the most auspicious dates for the worship of Lord Shiva in the Puranas. The worship of Shiva performed on these days brings great results. In fact, it is held that ashtami is not an auspicious day to undertake worldly activities.

In particular, the ashtami occurring in the krishna-paksha (dark fortnight of the lunar month) is an auspicious day for Bhairava, an aspect of Lord Shiva. Of all the monthly ashtami-s, the ashtami in the krishna-paksha of the month of Karthikai, is celebrated as Kalabhairavashtami, the day when Lord Shiva created Bhairava.

The form of Bhairava is symbolic of the notion that the outer appearance and adornment with things considered important in the world does not matter in a state of union with the supreme Shiva. Bhairava is the embodiment of supreme wisdom. Bhairava is worshiped as the form of wisdom in Kashmir Saivism.  Adi Shankara composed the Bhairavashtakam in praise of Bhairava, the giver of wisdom and salvation.

Those who worship Lord Shiva and the Kshetrapala - Bhairava on ashtami will be free of illness, enemies etc. and lead a good life. Lord Shiva also blesses them with supreme wisdom.

See Also:

• Kalabhairavashtakam
• Vaikasi Ashtami - Vrushabha Vratam 
• அஷ்டமி ப்ரதக்ஷிண மஹாத்ம்யம்