October to March is the cool, dry, low-humidity season and the most pleasant for sightseeing in the city and dry-land trips. The migratory birds arrive in the haors from November and stay until February or early March; the tea estates are green throughout the year but the workers and the picking are most active before the rains.
April to May is hot and dry — uncomfortable but not impossible. June to September is the monsoon, with heavy rains and frequent flooding. This is the only practical time to see Ratargul as a flooded swamp forest, and the rivers around Jaflong, Bichanakandi, and Pantumai Falls are at their most dramatic.
The annual urs (death anniversary) of Shah Jalal, held in the Bengali month of Joishtho (May-June), draws huge crowds to the dargah — worth attending if you don't mind the chaos, otherwise avoid.