The most important festivals in the Lao calendar for 2019:

January 01            International New Year Day
April 14 - 16         'Boun Pi Mai' Lao's New Year:
                               many activities along
                               the Mekong River
May 1                     International Labour Day
July 16                   Boun Khao Pansa Festival
                               (Buddhist Lent)
October 14            Boot racing festival
                               (Luang Prabang + Vientiane)
November 11        That Luang Festival, Vientiane:
                               religious festival at Phat That Luang
December 2          Lao National Day

The highlighted are the most important in the calendar. Some data change closer to the time. There are also a number of religious holidays that are celebrated mainly in the temples. The date is often dependent on the lunar calender.