Since 2010, I have seen a premium on two night EWR-ORD R/T fares in August. In 2009, the 2-day fare for Wed-Fri trip was about the same as Thurs-Sat. In 2010, 2011 & 2012, I paid a much lower airfare by traveling Thurs-Sat then I would've if I had traveled Wed-Fri.