Nearthling, any weekday is good. The crowds are gone and the uniform of the day is....Skin!
Do yourself a favor and explore upstream (my favorite!) or downstream, as the creek and the surrounding terrain changes constantly. I'm amazed that, over the years I've been making the hike, some of the folks I've met at the springs have never left the area of the creek crossing and the hot springs. There is so, so much more to experience there.