Nails and Hair represent Saturn.

Nail Cutters, scissors or any sharp object are ruled by Mars.

Cutting nails or hair require sharp object.

Tuesday is ruled by Mars. And Saturday is ruled by Saturn.

Both Mars and Saturn are not so friendly in nature. And cutting nails and hair on these 2 days bring them closer which create bad effect on your life. This is what I read somewhere online.

Other than that, I have no exact theory why doing same on Thursday is bad because many activities are not performed on Thursday. Like cleaning, buying anything, spending money etc. Because Jupiter is planet of expansion and whatever you do on Thursday will expand, so I observed Indians don’t go out shopping on Thursday. If you spend money on Thursday, your expense will increase.