Krishnanagar has a typical Gangetic plain climate: hot and dry from March to May (highs over 38°C / 100°F), heavy monsoon rains from June through September, and clear, comfortable winters from November to February. The best months to visit are November through February.
The town's biggest festival is Jagaddhatri Puja in late November, an enormous four-day celebration of the goddess Jagaddhatri that draws crowds from across Bengal. The town is decorated, the streets are full of huge clay images on their way to immersion, and accommodation is almost impossible to find without a long-advance booking. If you can manage it, this is the best week of the year to be here.