The theyyam season runs from October (the tenth day of Thulam in the Malayalam calendar) to late May. This is the primary reason most visitors come to the Payyanur area, and the timing of your visit should be built around it if theyyam is your goal.
Kerala's monsoon (June–September) brings heavy rains and is not ideal for travel, though the countryside turns intensely green and the backwaters fill to their highest levels.
The dry season from November to March offers the most comfortable weather for exploring — temperatures in the low to mid 30s°C, low humidity, and clear skies. By April and May it gets hot and humid before the monsoon breaks.
The Kavvayi backwaters are pleasant any time of year but are best in the early morning regardless of season.