Festivals and religious ceremonies are based on the lunar calendar. Its cycle is 29 days 12 hours 44 minutes 2.9 seconds, so 29,53 days). The 4 principal phases of the moon are considered as "Holy Days". Even if they are not public holidays, many sectors of activities do not work these days (all manual works), especially the full moons and the new moons.
Read more about Bouddhist Calendar on Wikipedia