Certainly you can have effects throughout the week, but the highest concentration, the highest frequency, is on the day you actually take the medication. It's simply the way the medication is absorbed etc. Pretty much most people are guaranteed vivid if not full-out nightmares on that day. When I was overseas it was Fridays. It could have been Mondays for somebody else, but definitely it was on that particular day.
On October 25th, 2016. See this statement in context.