In my opinion Monday and Tuesday are usually the worst days, because if a chica is going to take two days off, she'll try to take these days off because they are the slowest.
But yes you can find something all seven days of the week. Adelitas and Hong Kong seem to always have ladies around, but again in my opinion the numbers of girls is way down from my favorite days of visiting the zona, and that's Thursday or Sunday, as weekends are too crowded for me, but that's when you see the most chicas, but again any day is good, you'll find something.