The dry, cool months from November to February are by far the best time to visit. Daytime temperatures sit around 20-25°C, mornings can be cool enough for a jumper, and the skies are clear. October is good too, with the additional appeal of Chhath Puja -- the four-day sun-worship festival that turns the entire riverfront into one of the most extraordinary religious spectacles in India.
Avoid May and June, when temperatures regularly exceed 40°C and the city is uncomfortable. The monsoon (July to September) brings heavy rain and the risk of flooding from the Ganges; many years the lower-lying ghats are submerged. Prakash Utsav in late December or early January, the festival of Guru Gobind Singh's birth, draws huge Sikh pilgrim crowds to Takht Sri Patna Sahib.