During one of the evening conferences that Nancy Gilgoff held in Goa she talked about Saturday being a rest day and how this came to be.
When western students started arriving in Mysore to be taught by Pattabhi Jois, he found that he was teaching yoga in his shala every day from 5 am until about 2 in the afternoon. That was seven days a week, week after week.
His wife came to him one day and said that he had to take a break. She and the family were missing him and wanted to see more of him.
He decided that he would take Saturday off, because that was when all of the family would be available. The kids would not be in school or working.
In the Ashtanga yoga style you are encouraged to have a daily practice with one day of rest a week. Pattabhi Jois picked Saturday.
David Williams suggests that you take a day off a week as well. He found that Thursday works best for him.
A day of rest a week no matter which day it is. You should figure out which day of the week is best for you. Perhaps it will change from week to week.
Let me know which day of the week you pick.