Map from Google - Kolkata

In Kolkata (or Calcutta), the capital of the state of West Bengal, the climate is tropical, with a sunny and pleasantly warm winter from December to February (but sometimes the night temperature can drop below 10 °C: better to bring a sweater and a light jacket), a very hot period before the monsoon, from March to May (in which temperatures can touch or exceed 40 °C in the worst moments), and a rainy season from June to mid-October, characterized by a stifling weather, cloudiness and frequent rains. From mid-October to mid-November, before winter, there is a month characterized by good weather and high temperatures, around 30 degrees.
Here are the average temperatures.
Average temperatures - Kolkata
Min (°C)141721252627262626241914
Max (°C)262934363634323232322926

The monsoon goes roughly from June 5 to October 15, but already in May (and more rarely in April), the first showers in the afternoon may occur. The total annual rainfall is 1,600 mm. From November to April, the sun shines and it almost never rains.
Here is the average precipitation.
Average precipitation - Kolkata
Prec. (mm)132525451302603003052901603531591

From May to November Calcutta may be affected by cyclones, tropical storms from the Bay of Bengal, with a higher chance at the beginning and the end of the period, therefore in May and November). Bengal is particularly prone to flooding, due to the conformation of the territory (we are in the immense delta of the Ganges-Brahmaputra, and the river of the city, the Hooghly, is a branch of the Ganges).

Victoria Memorial Hall

South of Calcutta, the sea is warm enough for swimming throughout the year, although it drops to 23/24 °C in January and February.
Sea temperature - Kolkata
Sea (°C)232426282930292929292725