Basically, India has a three - season year - the hot, the wet and the cool. The best time to visit is during the winter (November through February), except for the northern Himalayan regions where late spring and summer (April through August) is the best time. See the climate charts in this section.
The Hot:
The heat starts to build up on the northern plains of India from around February, and by April or May it becomes unbearable. In central India, temperatures of 45 degree and above are commonplace - in the summer of 1994, Delhi had temperatures approaching 50 degree Celsius! It's dry dusty and everything is seen through a haze.
Later, in May, the first signs of the monsoon are seen - high humidity, short rainstorms, violent electrical storms, and dust storms hat turn day into night. The hot and humid weather towards the end of the hot season is like a hammer blow; you feel listless and tired and tempers are short.
The hot season is the time to leave the plains and retreat to the hills. Kashmir and the Kullu Valley come into their own, and the Himalayan hill stations and states such as Sikkim are at their best. The hill stations further south - Mt. Abu in Rajasthan, Matheran in Maharashtra, Ooty and Kodai Kanal in Tamil Nadu - are generally not high enough to be really cool but they are better than being at sea level. By early June, the snow on the passes into Ladhakh melts and the roads reopen. This is the best trekking season in northern India.
The Wet:
When the monsoon finally arrives, it doesn't just suddenly appear one day. After a period of advance warning, the rain comes in steadily, starting around 1 June in the extreme south and sweeping north to cover the whole country by early July. The monsoon doesn't really cool things off; at first you simply trade the hot, dry, dusty weather for hot, humid, muddy conditions. Even so, it's a great relief, not least for farmers who now have the busiest time of year ahead of them as they prepare their fields for planting. It doesn't rain solidly all day during the monsoon, but it certainly rains everyday; the water tends to come down in buckets for a while and then the sun comes out and it's quite pleasant.
The usual monsoon comes from the south - west, but the south - eastern coast is affected by the short and surprisingly wet north - east monsoon, which brings rain from mid - October to the end of December.
Some places are at their best during the monsoon - like Rajasthan, which has many Palaces built on lakes. The monsoon is also a good time to trek in the north - west Indian Himalayan regions, unlike in Nepal where the trekking season commences when the monsoon finishes.
The Cool:
Finally, around October, the monsoon ends, and this is probably the best time of year in India. Everything is still green and lush but you don't get rained on daily. The temperature are delightful, not too ho not too cool. The air is clear in the Himalayas, and the mountains are clearly visible, at least early in the day. As the cool rolls on, Delhi and other northern cities become quite crisp at night in December and January. It becomes downright cold in the far north, but snow brings India's small skiing industry into valley, have a winter season too.
In the far south, where it never gets cool the temperatures become comfortably warm rather than hot. Then, around February, the temperatures start to climb again, and before you know it you're back in the hot weather.