October to February is the best time to visit. Temperatures are manageable — typically 20–30°C — skies are mostly clear, and the light on the sandstone temples in the morning hours is exceptional. December and January are the coolest months and the most comfortable for walking the temple circuits.
March to May brings increasingly brutal heat. Temperatures regularly exceed 40°C by April and May, the sun is direct, and the pavements around the temples radiate heat back at you. If you visit in this period, limit temple time to early morning (before 9am) and late afternoon (after 4pm), and rest in the midday hours.
June to September: Tamil Nadu's northeast monsoon actually arrives late in the year, but a pre-monsoon season of occasional heavy showers affects Kanchipuram from June onwards. The full monsoon (northeast monsoon, unusual in that it peaks in October–November rather than August) brings flooding risk in low-lying areas and makes the roads to some temple sites briefly impassable. The town is quieter in June–September.
October–November overlaps the best weather with the northeast monsoon's peak, which can mean downpours. October is a slightly risky month; November generally clears.
Festivals
The Brahmotsavam festival at each of the major temples draws large crowds — tens of thousands of pilgrims — for ten-day cycles that vary by temple. Ekambaranathar Temple's Brahmotsavam typically falls in April–May, Kamakshi Amman's in February–March, and Kailasanatha's festival of Mahashivaratri (usually February or March) brings crowds to that otherwise quiet site. During these periods, accommodation books up and the temples become very crowded but also very alive.
Karthigai Deepam (November–December) is celebrated with oil lamps lit across the town and is visually spectacular.