February, March and April
Go nowDry, warm and not yet punishing
The best stretch of the year by a distance: dry days, warm rather than hot, low humidity and the sea calm enough to swim. Everything is open and nothing is flooded. Tết, the lunar new year, falls in late January or February and moves each year — the town is decorated and lively, but many businesses close for several days around it.
May
Go nowHot, dry and quieter than the spring
Still dry and now properly hot, with the beach at its best and fewer visitors than in the spring months. The old town is uncomfortable in the middle of the day, which is what the bicycle and the beach are for. Good value, and the last month before the real heat.
June, July and August
Worth it, butPeak heat, peak domestic season, and the beach
Hot and humid, with occasional short storms, and the busiest months for Vietnamese holidaymakers. The beach is the point now; the old town is a morning-and-evening activity. Prices rise and the coast is full. Perfectly workable if you plan around the middle of the day.
January
Worth it, butCool and grey, with the rains ending
The tail of the wet season: cooler than most people expect, often overcast, with the sea too rough for swimming and rain still possible. The flooding risk has largely passed, which is the main thing. Quiet and cheap, and the old town is pleasant to walk in cool weather.
December
Worth it, butThe wet season easing, but not over
Rain is receding and the worst flooding is normally behind, but it is still damp, grey and cool by Vietnamese standards. Late December brings a price bump over the holidays. A reasonable gamble, and much better odds than the two months before it.
October and November
Think twiceFlood season: the old town goes under most years
The northeast monsoon and the typhoon season arrive together, and the old town floods in most years — streets by the river under water, shops shut, boats where the pavement was. It recedes, but a short visit can lose everything. This is the clearest avoid window on the Vietnamese coast.
September
Think twiceThe rains start, and the typhoons with them
Still hot and now increasingly wet, with the first typhoons reaching central Vietnam and heavy rain building through the month. Boat trips and My Son start becoming unreliable. Cheap, and the beginning of the two months you actually want to avoid.