1

Hello
Yes, of course if you afraid rain or hot, the best is between Nov-Feb. However, to me all seasons are good to visit.
For instance, during rainy season, you have very clear sky (when it doesn’t rain) and nice landscape (green) with all rice fields growing and people working in the field.

However, what happens between Nov and Feb:

• 24 November 2008 : That Luang Festival and Trade Fair in Vientiane Capital

This religious festival is held in and around That Luang Stupa, the National Symbol of Laos, where hundreds of monks gather to accept alms and floral votives from the people. The festival includes a grand fireworks display at night. During the same period a similar festival is also celebrated at Ching Tim Stupa in Luang Namtha Province.

• Between mid December until January : Hmong New Year in all northern provinces and Vientiane (Vientiane in Feb usually)
• In February : Wat Phou Festival in Champasack Province

Festivities include elephant races, buffalo fights, cock fighting, and traditional Lao music and dance performances.
In February : The Elephant festival in Sayabouri Province

But if you like the hot temperature, in April, you should not miss the "Boun Pi Mai" Lao's New Year, between 13-15 April… in Luang Prabang, it is wonderful

In May you will have many Boun Bang Fai (Rocket Festival)
A ceremony praying for rain is performed at the temple in the morning. In the afternoon, people gather in fields on the outskirts of villages and towns to launch the rockets with much abandoned revelry. Villages, communities and departments compete for the "best decorated" and the "highest traveling" rocket.

And according to place, September or October, you will see the boat racing festival. In the evening, candlelight processions are held at temples and hundreds of colorful floats decorated with flowers, incense and candles are set adrift down the Mekong River to pay respect to the river spirit. The following day in Vientiane, Savannakhet and Champasack Province, a popular and exciting boat racing competition is held to celebrate the Mekong River.

Except for weather… it is difficult to say the best time.
In 2 weeks you have time to see more than Luang Prang and Vientiane.
Regards

2

For 2 weeks, I would not worry much about best time to visit. Avoid the holidays' rush of mid December to mid January...I say, go in late January.

3

we were in Laos mid may to late june, sometimes we had a rain shower in the afternoon. It was great weather. Rocket festivals at Champasak were great fun

4

If you can 'time' it, I'd recommend visiting Luang Prabang during the Boun Ok Phansa (End of Buddhist Lent) Festival which occurs in mid- to late October or the beginning of November. This is the 'mother' of all 'tak bat's. in LP.
They also celebrate Loi/Loy Krathong during the same festival (unlike in Thailand which celebrates later) wherein locals make their offerings to the 'naga' spirits with a massive gathering at the tip of the LP peninsula and especially with the focal point at the steps leading down from Wat Xieng Thong to the Mekong River. It's also where the 'boat offerings' / 'floats' made by the numerous villages that comprise LP and surrounding villages are set into the Mekong. I've attended 3 of the past 4 years and it just gets bigger and better. Only one caveat: in recent years some juveniles have taken to throwing fire-crackers so be aware and careful!
Festival-wise--From mid-Oct until the end of December (Hmong New Year) LP experiences the most religious and ethnic celebrations/events.