The best months to visit are February to May, when the weather is dry, temperatures are warm rather than hot, and the Old Town is at its most photogenic. February brings Tết, the Vietnamese New Year — the town is beautifully decorated with lanterns, but also busy with domestic tourists and some businesses close for a few days.
The rainy season runs from October to December, and floods can be severe: Hoi An floods regularly, sometimes covering the lower streets of the Old Town in half a metre of water. If you are visiting in this period, check conditions before you travel and keep hotel bookings flexible. The flooded Old Town is actually worth seeing if you can accept the inconvenience — the lanterns reflected in the street-level water look extraordinary.
July and August are the peak tourist months, coinciding with the summer school holidays in Europe and the school holiday in Australia. The Old Town is busy and prices rise.
The Hoi An Lantern Festival takes place on the 14th day of each lunar month — the full moon night — when electric lights in the Old Town are switched off and the streets are lit by lanterns. Worth timing a visit around if the calendar allows.