As far as I can tell from my limited tracking, it is now generally a huge department store/mall-wide "end of season" (what season?) sale time. Clothing that are generally overpriced even on American standards are marked down as much as 75%.
Warning, though, the lower marked clothing are not available to try on. So you either better be a very good guesser of sizing or a professional shopper to already know what fits. Being neither, I got pretty lucky when I bought three knit shirts ranging from $5 to $12 a piece.
No a bad deal given that the quality of department store goods tend to be better as the expression "you get what you pay for" truly applies in Bangkok. My $3 sandals bought in a stand in the weekend market have been, well, $3 quality. Which is why I bulk up on some cheaper quality goods, treating them as discardable consumables. If I weren't so uninterested in shopping at the moment, I would actually be browsing the department stores more to get some better clothing whilst they are on sale and still available.