The month decides the route more than the route decides the month
Four countries do not share a season. Something is wrong somewhere in almost every month of the year, and the job of this page is to say exactly what and where, so a date can be chosen with the trade-off known rather than discovered. The short version: mid-November to early February is the window where all four countries are dry at once.
Month by month, country by country
Vietnam is split because it has to be: the north, the centre and the south run on different calendars and the centre is the one that catches people out. Read your own month across the row, then read the four constraints below it, because a green cell in one column can still be ruined by a red one next door.
| Month | Vietnam | Laos | Cambodia | Thailand |
|---|---|---|---|---|
| January | North cold and grey; centre dry; south dry and warm | Dry and cool; cold at night; haze begins late in the month | Dry and cool — the best month of the year | Cool and dry; Andaman good; Gulf settling down |
| February | North cool and grey; centre dry from now; south dry | Dry; burning season under way in the north | Dry and warming; still very good | Cool and dry; open burning banned from 1 February |
| March | North warming; centre dry; south hot | Hot; burning season at its worst in the north | Hot, 35 – 40 °C at Angkor | Hot; Chiang Mai smoke peaks; islands unaffected |
| April | North warm and humid; centre dry; south hot before the rains | Hot; haze eases late in the month | Hottest month, 35 – 40 °C at Angkor | Hottest; Songkran mid-month; Gulf improving |
| May | North hot and wet from now; centre dry; south rains begin | Wet season begins; landscape greens | Rains begin; temperatures drop | Andaman monsoon begins; Gulf coast good |
| June | North hot and wet; centre dry; south afternoon rain | Wet; rivers rising | Wet, afternoon storms, few visitors | Andaman wet; Koh Samui at its best |
| July | North hot and wet; centre dry; south afternoon rain | Wet | Wet; Angkor green and quiet | Andaman wet; Koh Samui dry and bright |
| August | North hot and wet; centre dry to mid-month; south afternoon rain | Wet; Mekong high | Wet | Andaman at its wettest; Koh Samui still good |
| September | North rain easing; centre typhoon season opens; south wet | Wet; rivers high | Wettest month; moats full, stone greenest | Andaman wet; Gulf beginning to turn |
| October | North dry and pleasant; centre floods; south drying | Rain ending; Mekong high and navigable | Wettest; green, quiet, warm | Andaman rain ending; Koh Samui wet from now |
| November | North dry and cool; centre still floods early; south dry | Dry and cool — the best season opens | Dry; the season opens | Cool and dry; Koh Samui at its wettest |
| December | North cold and grey; centre drying; south dry | Dry; cold nights in the north | Dry and cool; peak season | Cool and dry; Andaman good; Koh Samui wet |
The four constraints that actually decide a date
A table of averages hides the things that ruin trips. Four constraints do most of the damage, and each of them is specific enough to plan around.
One. Central Vietnam floods from September to November
Hue and Hoi An sit on the wrong side of the Truong Son range for the northeast monsoon, and their wet season is not the same as the rest of the country. Typhoon and flood season runs September to November and is worst in October and November. Hoi An floods most years — the old town takes water through the ground floors, boats replace bicycles on Bach Dang street, and the heritage houses close. The 120,000 VND old-town ticket is irrelevant when the buildings you are entitled to enter are shut.
This matters far beyond central Vietnam, because almost every Vietnam route runs Hanoi to Hue to Hoi An to Saigon. October is a good month in Hanoi, a good month in Saigon and a bad month in the 250 km between them. Where dates cannot move, the route can: we run the north and the south and replace the centre with the Mekong Delta or Phu Quoc, and we say so before you book rather than while you are packing.
Two. The burning season in northern Thailand and northern Laos
From late January to late April, farmers and forest fires fill the valleys of northern Thailand and northern Laos with smoke that does not clear until the first rains. March is the worst month. Chiang Mai recorded PM2.5 of 188 µg/m³ on 29 and 30 March 2026, twelve times the WHO guideline, with 158 wildfires burning across fifteen districts on 24 March alone. Thailand banned open burning from 1 February to 31 May 2026, which helps at the margin and does not clear the air.
The visible effect is that the mountains around Chiang Mai and the ridges above Luang Prabang disappear behind a flat white haze. The invisible effect is what it does to anyone with asthma or a heart condition, and to children. We do not send clients to Chiang Mai in March. Where a March or early April journey wants northern Thailand, we substitute the Gulf coast or the Andaman, both of which are unaffected.
Three. The Gulf coast runs backwards
Thailand has two coasts and they are on opposite calendars. The Andaman side — Phuket, Krabi — takes the southwest monsoon from May to October and is dry from November to April, which matches everyone’s mental model of Thailand. The Gulf side does not. Koh Samui is at its best from June to August and at its wettest from October to December, precisely when the rest of the region is entering its best months.
The consequence is awkward and worth stating early: a journey timed for the four-country window in late November or December should put its beach on the Andaman, not on Samui. A journey in June or July, when the mainland is wet, can put a genuinely dry week on Samui at the end. Choosing the island first and the dates second is the most common way a honeymoon ends up sitting out a monsoon.
Four. Heat at Angkor against water at Angkor
Angkor is at 35 to 40 °C from March to May. Temples are stone, courtyards have no shade, and the walk from the Angkor Wat causeway to the upper level in that heat is genuinely unpleasant for anyone over sixty or under twelve. The park runs 07:30 to 17:30, Angkor Wat opens at 05:00, and in hot months the working day is the two hours after opening and the two before closing, with the middle of the day given back to the hotel.
The wettest months, September and October, are the opposite trade. It rains hard in the afternoon and rarely all day, the moats are full, the stone is at its greenest and the crowds thin out. The photographs people admire of Angkor are mostly taken then. What you accept in exchange is mud on the Ta Prohm boardwalks and a real chance of losing an afternoon. Siem Reap in November through February avoids both problems and is priced accordingly.
The answer, stated plainly
For a three- to four-week journey across all four countries, the window is mid-November to early February, and the sweet spot inside it is late November to mid-January. That is the only stretch of the year when northern Vietnam is dry, central Vietnam has stopped flooding, Cambodia and Laos are in their dry season, and northern Thailand has not yet begun to burn. Every route on this site is built to run best in that window.
What you trade for it
- Grey cold over Ha Long and Lan Ha. December to February brings low cloud and 14 to 18 °C on the water. The bay is quieter and the light is flat; photographs will not look like the brochure. The cruise still runs, but bring a jacket and expect the horizon to be missing.
- Christmas and New Year pricing. Hotel rates rise 15 to 40 per cent from around 20 December to 3 January, and the better properties impose compulsory gala dinners on 24 and 31 December that are charged whether or not you attend. Moving the same journey two weeks either side removes most of that cost.
- Cold nights in Sapa and northern Laos. Sapa can drop to 5 °C and lower with fog that closes the valley views for days; Luang Prabang and the Lao north are pleasant by day and genuinely cold before dawn, which matters at the alms round, which runs 06:00 to 07:00 from November to February.
- Demand. This is peak season across four countries at once. Cruise cabins, Lao train seats — station windows open only three days ahead and second class sells out — and the best hotel rooms go six to nine months in advance.
Second best, and worst
Second best: October, everywhere except central Vietnam
October is an underrated month if the route is built for it. Hanoi is dry and pleasant, Laos has stopped raining and the Mekong is high enough for the slow boat, Angkor is green with full moats and few visitors, and the Andaman coast has come out of the monsoon. Only central Vietnam is unusable, and only Koh Samui is entering its wet season. Prices sit below the November to January band and hotels have availability. Build an October journey around the north of Vietnam, Laos, Cambodia and the Andaman and it is close to the best trip of the year.
Worst: March and April
March and April are the months we most often advise moving. Angkor is at 35 to 40 °C, the whole of northern Thailand and northern Laos is under smoke with March at its peak, Vietnam is between seasons, and Songkran in mid-April closes businesses and fills domestic transport for the better part of a week. There is a version of a March journey that works — southern Vietnam, Cambodia early in the morning, the Gulf or the Andaman — and it is a two-country trip, not a four-country one.
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Questions travellers ask most
Is there any month when all four countries are good?
Mid-November to early February, and nothing else comes close. Late November to mid-January is the strongest part of it. Outside that window at least one of the four is wet, burning or too hot, which is why routes built for other months tend to drop a country rather than stretch across all of them.
How bad is Hoi An in October, really?
Bad enough that we redraw the route. The old town takes water through the ground floors most years, the heritage houses close and the streets flood for days at a time. It is not a matter of an umbrella. September is a gamble, October and November are the peak of the flood season, and by late December the town is dry and pleasant again.
Can we still go to Chiang Mai in March?
You can, and we would rather you did not. PM2.5 in Chiang Mai reached 188 µg/m³ on 29 and 30 March 2026, twelve times the WHO guideline, with 158 wildfires across fifteen districts on 24 March. The mountains are invisible and the air is genuinely harmful for children and anyone with a respiratory condition. Bangkok and the coasts are unaffected.
When should we go if we want a beach at the end?
It depends entirely which coast. Between November and April put the beach on the Andaman, at Krabi or Phuket, or on Phu Quoc in Vietnam. Between June and August put it on Koh Samui, which is dry then and wettest from October to December. Choosing the island before the dates is how honeymoons end up in the rain.
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