Saturdays and moon days.... I hadn't the foggiest idea what the meaning behind these specific days were until I read over some blogs.
Saturdays are reserved as days to relax and recuperate from the weeks practice. Traditionally, a castor oil bath is completed on this day to remove the heat that builds up over the week inside of the body and after the bath the body is extremely relaxed and it is advised to take the day very easy. Not only is it advised to take Saturday easy but it is also advised to take Sunday's practice easy as well because the body will be still affected by the oil and it is easy to over stretch during this time. There are quite a few blogs out there that dictate how to use castor oil for a bath much like Claudia's from Ashtanga Yoga Mother Earth, she put this together, and it explains her process of the Saturday practice castor oil bath.
In regards to the moon days, the practice is 6 days a week, therefore not many days off from practice. Since our bodies are composed of primarily water, we are affected by the moon just like the ocean is affected by it. Our bodies feel the ebb and flow from the pull of the moon and days that the mood is either a new moon or full moon, our bodies feel it the most. Therefore, we rest on those days when the moon has the most effect on our bodies. Tim Miller gives a great explanation of why yogis don't practice on Moon Days in Ashtanga Yoga here.
While I'm not completely knowledgeable in either of these categories (especially the castor oil bath) I find them equally interesting. I intend to find out as much as possible about the history of both of these practices. When I come to an understanding I'll be sure to post an update.