Dawn or late afternoon are the only sensible times to climb. The granite of the staircase and the summit rock heats brutally in the middle of the day, and there is little shade at the top. Most pilgrims start before sunrise.
Poson Poya in June (the full-moon day commemorating Mahinda's arrival) is the big day in the Sri Lankan Buddhist calendar — Mihintale fills with white-clad pilgrims through the night and the whole hill is lit with oil lamps. It is genuinely spectacular but extremely crowded; expect long queues on the staircase. The dry season runs from May to September on this part of the island; the wetter months from November to January bring heavy afternoon showers but the cooler mornings are pleasant.