The best time to visit Ooty is April to June, when the hills are green, the days are clear and pleasantly cool (15–20°C by day), and the Nilgiri Railway operates its full timetable. April and May are peak domestic season — expect crowds at the main attractions and book accommodation in advance.
The southwest monsoon arrives in June and brings heavy rain from the west through October. The hills can be dramatic in the mist, and room rates drop significantly. October is particularly good for the Mukurti plateau, when the rains have gone and the grasslands are at their greenest.
Avoid the December–January school holidays if crowds bother you. February and March are the quietest months — cooler than ideal but uncrowded.