Kalashtami is the term given to the eighth day of the dark half of every lunar month. This day is dedicated to the Kaal God, generally known as Kaal Bhairav.
Kal Bhairava is another Avatar of Lord Shiva. Devotees of Kalbhairva Avatar of Lord Shiva observe fast and perform Puja on this day. Kalbhairav Jayanti is celebrated in all over India. As India follows lunar calendar, some regions of India use Poornimant calendar and some uses Amavasyant calendar. Poornimant is that lunar calendar which is considered from the full Moon night and ends at full Moon night. On the other hand, Amavasyant calendar is the one which starts from new Moon night and ends at same new Moon night.
People of North India follows Purnimant Calendar, therefore they celebrate the main Kalashtami in the Margashira month. On the other hand, followers of Amavasyant calendar celebrate it in the month of Kartik