According to my Thai boyfriend, Sundays are the lightest travel day by far: Mon thru Fri is predictably busy during morning and afternoon rush hours, and Saturday is his least favorite, since while there are plenty of people on the road, it's "Unpredictable". Sunday? No problem--at least no reason to change your flights.
Also, at least as of last month, the new Airport Link train is only operating Mon thru Fri, 7-10AM and 4-7PM.
Someone else probably knows more than me, but I'd check on that before I made any schedule changes!