The best time to visit Thanjavur is between November and February, when temperatures are mild — highs around 28–30°C — and humidity is bearable. The northeast monsoon brings rain from October to December, which can be heavy; the temple complex becomes slippery and some days are uncomfortable, but crowds are thinner.
The summer months (April–June) are genuinely harsh, with temperatures climbing above 40°C. Most visitors sensibly avoid this period unless they have a specific festival reason to come. The Thai Pusam festival in January–February and the Brihadisvara Temple's annual festival in the Tamil month of Karthigai (November–December) are worth timing a visit around if you have any interest in temple ritual.