You can ALWAYS find plenty no matter what night of the week it is. Monday-Sunday, Insanity, Mixx, Spicy, all the gogos, etc. will be filled with women.
If you're looking for "normal" girls that go with farangs but aren't technically "working girls," then Thursday-Saturday will be your best. The venues I'm talking about include Levels, Q-Bar, etc.
Fun to be had 7 nights a week in Thailand:)