When is Kali Puja Celebrated?

Kali Puja is celebrated on the Amavasya or the no moon night in the Hindu month of Kartik (October/November). This date of Kali Puja coincides with Diwali or the Festivals of Lights.
Legend Behind Kali Puja
According to legends once the demons named Shambhu and Nishambhu grew in force and pose a challenge to Indra, King of Gods, and his Kingdom of Heaven. Gods sought protection from Mahamaya Durga, the Goddess of Shakti or Power.At this stage Goddess Kali was born from Durga's forehead as Kal Bhoi Nashini to save heaven and earth from the growing cruelty of the demons.
After slaughtering the demons, Kali made a garland of their heads and wore it around her neck. In the bloodbath, she lost control and started killing anyone who came her way. There was chaos all around. To stop her, Lord Shiva threw himself under her feet. Shocked at this sight, Kali stuck out her tongue in astonishment, and put an end to her killing spree. The well-known picture of Kali Maa shown with her tongue hanging out, actually depicts the moment when she steps on Lord Shiva and repents.
That momentous day is celebrated ever since as Kali Puja. Performing the Puja with faith devotees seek blessings for general happiness, health and prosperity. Kali Pujan is a tantrik puja performed only at midnight on Amavasya.
Kali Puja Celebrations in West Bengal
Kali Puja is one of the major festival for people in West Bengal and they celebrate the occasion with much gusto and enthusiasm. Just as people in North India light lamps to honour Lakshmi Maa, people in West Bengal celebrate Kali Puja by lighting lamps in honour of Goddess Kali. Houses are decorated and elaborate Rangoli pattern are drawn in front of houses and courtyard. Elaborate Kali Puja is carried out during the evening. Through Puja people seek happiness, prosperity and protection against hardships.