The dry zone of the north-central plain is hot year-round, but the most comfortable months for visiting are December to March, when daytime temperatures hover around 30°C and humidity is lower. April can be brutally hot — over 35°C is common — and from May to September the southwest monsoon brings occasional rain. The inter-monsoonal period in October and November can be unsettled but is greener.
Try to time a visit around the full moon (Poya) days, when the sacred sites fill with white-clad pilgrims and the atmosphere becomes extraordinary. Esala Poya in July or August is particularly significant; the Sri Maha Bodhi receives long processions and the dagobas are illuminated. Avoid arriving in the heat of the afternoon — the ruins are largely without shade, and a 6 AM bicycle start is the standard approach for serious visitors.