Kalashtami (Kalabhairavashtami) is an auspicious vrat dedicated to Lord Shiva, observed on the Ashtami after the Purnima in Hindu calendar i.e. Krishna Paksha Astami.
Pitru Tharpan and Pinda daan (special rituals to dead ancestors) are also performed to please the dead souls.
Kalashtami vrat is observed mainly in Maharashtra and some other western parts of India. Maha Kalbhairav Jayanti (Mahakaal Bhairav Ashtami) is the most auspicious Kalashtami as it is the Jayanti of Lord Kalbhairav.
It is observed in Karthik Month as per Marathi calendar & Margashirsha Month as per Margashirsha Month. Here is the list of Monthly Kalashtami dates in 2012..
Kalashtami in Poush Month: January 16, 2012, Monday
Kalashtami in Magh Month: February 14, 2012, Thursday
Kalashtami in Falgun Month: March 14, 2012, Wednesday
Kalashtami in Chaitra Month: April 13, 2012, Friday
Kalashtami in Vaishakh Month: May 12, 2012, Saturday
Kalashtami in Jyeshta Month: June 11, 2012, Monday
Kalashtami in Ashada Month: July 11, 2012, Wednesday
Kalashtami in Shravan Month: August 9, 2012, Thursday
Kalashtami in Adhik Bhadrapad Month: September 8, 2012, Saturday
Kalashtami in Bhadrapada Month: October 8, 2012, Monday
Kalashtami in Ashwina Maas: November 7, 2012, Wednesday
Kalashtami in Kartika Maas: December 6, 2012, Thursday – Kalbhairav Jayanti