February to May is the sweet spot, with warm, dry, sunny days before the fierce summer heat sets in. June to August is very hot and humid but still dry, while September to December brings the rainy season and, in October and November, the Thu Bon River can flood and briefly submerge parts of the old town. Time your visit around a monthly full-moon lantern night if you can.
In the wet season, roughly September to December, heavy rain and river flooding can put low-lying streets of the old town under water, sometimes for days. Check the forecast in those months, avoid wading through floodwater, and follow local advice on which areas to skip.
Away from the car-free old town, roads to the beach and villages carry fast motorbike and car traffic with informal crossings. Take care when cycling or walking on the main roads, wear a helmet on any motorbike, and cross with awareness rather than assuming vehicles will stop.
Summer sun is strong and open sites like the My Son ruins and the beaches offer little shade. Carry water, use sun protection and a hat, and visit ruins and outdoor markets early in the day to avoid the worst of the midday heat.