I was traveling in Phayao province this past week and was following a tourist sign to Ban Huak Border Market. (see map)

Apparently it is only open on one day during the month. I should have asked when but didn't.

Does anyone know what day it is open?

My best guess would be 'wan lang wan sin-duan', the day after pay day, end of the month.

It used to be open every week-tue i think, but long time ago. A nice hot water water fall close to.

I stopped at the waterfall and followed their 1km trail through the jungle. It's a very nice walk and peaceful.

Is there anything special about this market?...other than being near a border?

I haven't been to the market myself, but according to the PN map of Northern Thailand (which often isn't accurate) the Ban Huak market opens on the 10th, 20th, and 30th of every month. I've learned (the hard way) not to completely trust PN Maps, so perhaps you could go with a "plan B" in mind (i.e. visiting Pu Sang Waterfall or Phu Chee Fah if the market is not open as advertised.)

Perhaps you could post some photos of the market for us, if you get lucky!