Today happens to be the Kaal Bhairav Ashtami, also known in some parts as Kalashtami or Mahakaal Bhairavashtami. This special day is dedicated to the Lord Kaal Bhairava–a manifestation of Lord Shiva. The ashtami falls on the eighth lunar day in the fortnight of the waning moon (Krishna paksha) in the Hindu month of Kartik (per the South Indian Amavasyant calendar, every month ends with a new moon) or Margashirsha (per the North Indian Purnimant calendar, every month ends with a full moon).
As per the legendary tale, in the form of Mahakaleshwar, lord Shiva cut off the fifth head of Brahma and performed penance on this day. However, there are other legends too which define the reason why Brahma's fifth head was chopped off.
Another one goes that lord Brahma insulted Shiva, and after that Bhairava (Kala-Bhairava) appeared , which was a manifestation of Shiva's (angry) forehead and severed Brahma's head. The head of Brahma stuck to Bhairava's left palm due to the sin of killing Brahma, the most learned Brahmin – Brahmahatya or Brahminicide. In order to perform the penance of this sin, Bhairava had to perform the vow of a Kapali: wandering the world as a naked beggar with the skull of the slain as his begging bowl. Bhairava's sin is finally expiated when he reaches the holy city of Varanasi, where a temple dedicated to him is situated.
It must be noted that on this day, those who worship the lord take bath in the morning and perform pujas. Special puja for ancestors is also performed by the devotees in the morning. Some even keep stay awake all night, and narrate the stories of Mahakaleshwar.