The Cool Season runs from late Oct or early Nov to around mid-February. This is the best time to go (though also the most crowded):

Days are pleasant (in the 20s C), nights can be cold (down to freezing in the high-altitude far North, but more like 10-12 C in Luang Prabang and other tourist hot spots). 10-12 may not sound cold, but after sitting still for several hours, with no buildings being heated, it’s cold. Bring layers.

Early in the Cool Season (late October / November) right after the monsoon, the country is still very green and lush, skies are free of haze, but many roads and trails will still be muddy and you will still encounter a lot of mosquitoes.

The best weather is in the middle of the Cool Season, right around Christmas and New Years. This is also the most crowded (but still far less crowded than Thailand).

By January, mud and mosquitoes are almost entirely gone. By late January the afternoons start getting hot, the green scenery is turning brownish, and the air starts getting dusty and hazy. You may see a little burning in northern Laos by late January, but it isn’t enough to affect anything.

Lunar New Year / Chinese New Year / Tet takes place during late January or February (the date changes year to year). Although Lao New Year isn’t until April, a lot of Chinese and Vietnamese tourists flood in at this time and lodging can get scarce and expensive specifically in Luang Prabang. I was in Vang Vieng dring that holiday in 2023 and I noticed no impact whatsoever, however in 2024 a number of tourists mentioned big price spikes and por availability in LP.

Weather gets slowly but steadily hotter and hazier throughout February. By late February, trekking becomes uncomfortably hot. Up north, you’ll increasingly see the hill tribes burning their rice paddies in preparation for spring planting, which makes it worse. But the burning and its consequent smog usually only get really bad in March.

Beware the Hot Season in March, April and early May:

It is REALLY hot (highs above 40 C are common), and in northern Laos the smog from the burning of the rice paddies will burn your eyes and throat. You can’t see more than a few metres ahead, it’s hard to breathe, and it’s extremely bad for your health. Late March is the worst for the burning (avoid northern Laos entirely then if you can), and then it starts to dissipate and the smoke is usually gone by Lao New Year (called Pi Mai or Songkran, from 13-15 or 14-16 April). But it will still be hot as hell all over the country until monsoon starts, so definitely not a good time for trekking. The main reason I can see to come at this time is to enjoy the fun New Year festivities.

Monsoon runs from around mid-May to late October:

During the monsoon, when it’s not raining you can expect haze-free skies, and lush landscape, and the lighting when the sun comes out can be stunning. Or, you might get unlucky and it might rain non-stop for days.

As a very general rule, from the start of monsoon until late July / early August it typically rains for a few hours on most days, but you also have long periods of sunshine each day. But from then until the end of monsoon sometime in Otober, it is more likely to rain non-stop. But, that is a huge generalisation. The simple fact is that in monsoon you might get lucky, or you might not.

You will also have very high humidity and clouds of mosquitoes. Malaria is very rare in Laos, but occasionally in the most remote areas there might be some. If you can take malaria pills without having bad side effects, you might as well do so - it can’t hurt. But if that causes you any side effects, I would just use DEET.

On any wilderness hikes during monsoon, expect your legs to get covered in leeches as you sink into the mud.

Most unpaved roads are not drive-able during monsoon, and that puts a lot of the country off-limits.

But if you only use paved roads and make allowances for everything I’ve already mentioned, you may be one of many who has a wonderful time with no crowds at all.

Go to https://lpb-air.com/ for detailed data on air quality in Luang Prabang, both current and historical. You can see exactly what air quality to expect on any given date.