October to March is the best time to visit Kolkata, and within that, Durga Puja (late September or October) is the single most compelling time to come if you can handle crowds.
Durga Puja is the reason many people time a trip to Kolkata. Inscribed on UNESCO's Representative List of the Intangible Cultural Heritage of Humanity in 2021, it turns the entire city into an open-air exhibition. Thousands of pandals — elaborate temporary structures housing idols of Durga — are built across every neighbourhood, many designed by professional artists and architects around themes that range from traditional to sharply contemporary. They are free to enter, open through the night, and the crowds walking between them are enormous and good-humoured.
The main days run from Shashthi to Dashami, five days of continuous activity, ending with the immersion processions on Vijaya Dashami. Accommodation books out months ahead and prices rise sharply. Roads are closed and moving around is slow. It is worth all of it.
November to February is the ideal window for a conventional visit — pleasant temperatures, low humidity, dry weather, and the city comfortable to explore on foot. December and January are the coolest and most agreeable months. This period also carries the Kolkata Book Fair (usually late January to early February), one of the largest book fairs in the world, and the Kolkata International Film Festival.
Avoid April to June. Heat combined with humidity is genuinely draining, and it limits what you can do.
July to September is manageable if you accept rain and waterlogging. Prices are lower and crowds thinner. The city looks different in the rain, and some find it atmospheric, but plan flexibly.