This is because in ancient Thailand, Wednesday used to be considered the day of agriculture, a day of growing things. Hence, as cutting is the opposite to growing, it was thought it would bring bad luck.
Kannikor Rubngarm, a 28 year-old from Bangkok, also remembers her grandfather’s strict thoughts on the matter.
“He believed that cutting hair on Wednesdays was inauspicious, and thought it would bring bad luck. He often said that you would perhaps fight and quarrel with someone as a result.”
Two generations ago, Thais often thought that ‘trouble’ or weird incidents that occurred could be directly traced back to when they had had a haircut on a Wednesday.
- See more at: http://www.thephuketnews.com/the-cutting-truth-why-you-cant-cut-your-hair-on-a-wednesday-41739.php#sthash.IbQAVGbn.dpuf