Sunday is a day of rest in our house; not a religious practice, more a practical way to rejuvenate and prepare for the next week of work.
R & R for the soldiers in the military is meant as a vacation away for a couple weeks, and usually there is no Rest or Relaxation during that two weeks.
There is a bit of information here about relaxation:
"Here are some ways to recover from stress: proper sleep of course is essential, but meditation, yoga, listening to music or just having a nice warm bath are great for counteracting stress, it slows down the body and reduces fear, anxiety and tension."
So Sunday is a day off work, a day to rest, to recoup, to prepare for another week.