The single most important factor in timing a visit to Thrissur is the Thrissur Pooram, held at the Thekkinkadu Maidan in April or May during the Malayalam month of Medam. The exact date changes each year but falls before the monsoon. The festival assembles over 100 caparisoned elephants, runs percussion ensembles of extraordinary complexity, and culminates in a competitive fireworks display between two rival temple groups that begins at midnight and runs until dawn. Accommodation in the city fills months in advance; book as early as possible and plan to stay the night.
Outside Pooram season, the best time to visit Thrissur is October to March — the post-monsoon period when the hills are green, the air is cool by Kerala standards (25-28°C), and the waterfalls at Athirappilly are full without being at their most dangerous. The monsoon (June-September) can be spectacular — Kerala's rainfall is dramatic — but roads to the waterfalls and forest areas may be closed.