The Cauvery Delta is hot year-round, but the worst of the heat is from April to June, when temperatures regularly hit 40°C. The northeast monsoon brings heavy rain from October through December, and the delta's flat terrain means waterlogging is common during the worst weeks.
The best window for a visit is January through March. The weather is warm but manageable, the post-monsoon landscape is green, and — crucially — the Panguni Brahmotsavam falls in March or April. If you want to see the Rajagopalaswamy Temple at its most alive, time your visit for this 10-day festival. The Garuda Sevai procession (typically on the fifth day) is the highlight. For birdwatching at Thiruparkadal Lake, come between November and February, when migratory species are present.