Siena is at its most intense around the two Palios — 2 July and 16 August. The days leading up to each race fill the city with parades, blessing rites, contrada dinners, and trial runs in the Campo. Hotels book months ahead and prices triple. If the experience itself is the goal, this is the time; if you want to see the buildings, come at any other time.
Spring (April–June) and autumn (September–October) are ideal — mild weather, long light, fewer day-trippers from Florence. Winter is quiet, sometimes damp, and the museums are nearly empty; this is when the city feels most lived-in. August outside the Palio is fiercely hot and crowded.