In northern Laos, including Luang Prabang and Vientiane, November to mid February is the 'cool' season - dry and quite warm, an ideal time to come. Mid February to May is the hot season and out in the rice fields the lushness disappears, but 85% of Laos is forested mountain, so it's still green. In March and April, smoke from agricultural fires can be a nuisance. The rainy season is May to late October - hot, humid and wet, especially in June-July; but Luang Prabang gets much less rain than Vientiane.
13-15 April: Lao New Year (Pi Mai) is celebrated with great gusto - folk songs, circle dances and and candlelit processions - particularly in Luang Prabang, where effigies of Naga (protector of the city) and Nang Sang Kham (guardian of the new year) are carried into town from the stupa on Mt Phusi.
May (date varies): Rocket Festival (Boun Bang Phai) where bamboo projectiles are fired into the sky to bring on the rains, and those whose launches fail are thrown into the mud.
August (Luang Prabang)/September (other locations): Boat races on the Mekong river, with crews of up to 50 men and women, to celebrate the end of the rainy season.
October (Vientiane): Boat races take place on the river to celebrate the Bun Nam Water Festival.
November (Vientiane): Boun Phra That Luang is celebrated, a 3-day religious festival at That Luang every year when hundreds of monks accept alms and flowers from the people.
NB, please do not rely solely on this information for your travel planning.
LUANG PRABANG: BY AIR
There are no long-haul flights into Luang Prabang. A few international flights connect neighbouring countries: Lao Airlines, Bangkok Airways, Vietnam Airlines.
By bus from Thailand: From Chiang Rai, you can travel by bus or car to Chiang Khong on the Mekong river (the border), and then take a boat across to Houei Xai in Laos. From Houei Xai there are boats down river to Luang Prabang (1-2 days), but there are no schedules or safety enforcements, so you should be prepared for an adventure. We'd recommend Luang Say Cruise, a leisurely overnight trip.
VIENTIANE: BY AIR
There are no long-haul flights into Vientiane but you can connect with flights. Try Air Asia, Lao Airlines, Thai Airways, Lufthansa, Vietnam Airlines, Lao Airlines, Vietnam Airlines.
CHAMPASAK PROVINCE: BY AIR
Fly to Pakse International Airport then try Lao Airlines.
NORTHERN LAOS: BY AIR
Fly to Oudomxay Airport then try Lao Airlines.
Malaria is prevalent in Laos and you are advised to take prophylactic medicine, preferably something other than chloroquine as there is a high level of resistance.
Other advisable vaccinations include hepatitis A, tetanus, polio and typhoid (these last 3 may be covered by previous inoculations or boosters which you have had). HIV is widespread.
There are occasional incidences of dengue fever (hard to protect against), and avian flu seems to be an increasing risk (keep informed).