India does not have a travel season. It has about five, running simultaneously in different places. The question is never "when should I visit India" — it is "where is worth being in the month I can travel".
January–February
Go: Andamans, Hampi, Coorg, Araku, Kerala, Rajasthan. Ladakh for snow leopard tracking.
Peak season across the south, and the only comfortable window for Hampi — by March the open granite passes 38 °C and the ruins become unpleasant by mid-morning.
Avoid: Himalayan high passes. Spiti is reachable only via Shimla; Kunzum La is shut.
March–April
Go: Sikkim and Kashmir for rhododendron and almond bloom. Kedarkantha, which still holds summit snow with a fraction of the December crowds. Coorg before the heat.
The shoulder month nobody uses properly. Northern hill stations are open, southern plains are not yet punishing, prices are lower.
Avoid: the plains from late April. Delhi, Rajasthan and central India start becoming genuinely hard.
May–June
Go: high Himalaya. Spiti's Manali road opens around late May. Kashmir's meadows come alive. Valley of Flowers opens 1 June.
Avoid: everywhere below 1,500 m. This is the hardest month on the plains.
July–August
Go: Spiti and Ladakh — both sit in rain shadow and stay dry while the rest of the country floods. Valley of Flowers hits peak bloom mid-July to August. Hampta Pass works for the same rain-shadow reason. Kashmir Great Lakes.
The monsoon is not a reason to stay home; it is a reason to go north of the main Himalayan range.
Avoid: Meghalaya trails, coastal Karnataka and Goa for walking. Spectacular to look at, difficult and leech-ridden to walk. Andaman ferries are cancelled regularly.
September
Go: Sikkim as the monsoon clears. Ladakh before the passes close. Western Ghats for post-monsoon waterfalls at full flow.
Avoid: anywhere on a tight schedule — September is when landslides most often close Himalayan roads.
October–November
Go: almost everywhere. The best month in India if you have a free choice. Meghalaya opens up, Coorg and Araku are green with waterfalls still running, Sikkim gives the clearest Kanchenjunga views of the year, the Andamans settle.
Avoid: nothing much. Book early — everyone else has worked this out too.
December
Go: Kedarkantha and the winter trek season. Andamans, Kerala, Hampi, Rajasthan. Gulmarg for snow.
Avoid: North Sikkim, where roads become unreliable. And be honest about cold — Spiti in December means −20 °C in Kaza.
The rule underneath all of this
Altitude beats latitude. When the plains are unbearable, go up. When the mountains are closed, go south. India is large enough that something is always in season.
The mistake is picking a destination first and then discovering it has a three-month window that does not match your leave. Start with the month.