Kovilpatti is dry and hot for most of the year. The most pleasant months are November through February, when daytime temperatures sit around 30°C and nights cool off. March to May is uncomfortably hot, with the inland Tamil Nadu sun pushing temperatures into the high thirties.
The northeast monsoon brings the bulk of the rain in October and November. If you can time a visit to the Shenbagavalli Amman Temple festival around Tamil New Year (mid-April), the town is at its most lively, but the heat is also at its worst.