Colombo is hot and humid year-round (27–32°C), so there is no single best season. The southwest monsoon (May–September) brings heavy rain, especially in the afternoons; the northeast monsoon (November–January) is lighter and affects the north and east of the island more than the capital. The driest months are January to March and July to August.
For a Colombo visit combined with the west and south coasts, December to March is best. For the cultural triangle (Kandy, Sigiriya, Anuradhapura), July to September works well. The city celebrates Vesak (Buddha's birthday, full moon in May) with extraordinary illuminations — the whole city fills with lanterns and elaborate light installations and it is worth timing a visit around.
Ramadan affects the Pettah and Muslim areas of the city noticeably — many restaurants close during the day, but the evening iftar meals are lively and worth experiencing.