The city of Kolkata witnesses a rainy period almost throughout the year. So much so that the city records some amount of rains every month. The least rainy months happen to be December and January, out of which December witnesses rain in single digit.
Kolkata gets rain during the Southwest and Northeast Monsoon and even after that as and when weather systems develop in the Bay. In Kolkata, the withdrawal of Southwest Monsoon usually happens by the end of October, which sometime overlaps with the arrival of Northeast Monsoon, thus, rainy days continue, and the city remains active in terms of rainfall.
During the month of December, if and when Kolkata receives rains it is either due to the oceanic systems forming in the Bay or the winter systems that approach the Himalayas. Rains get amplified when these systems coincide and appear at the same time.