This will be my 3rd year at the sale. The prices really vary but I would say you can expect at least 40% off everything - some things are awesome deals.
They start the first day with higher prices - but you get a better selection. By the last day they have things marked to move.
I went on Day 2 - two years ago - got some great deals and my choice of patterns. My sister went back on the last day and got smokin' deals but not a great selection
Last year I went on Day 3 - great deals and lower selection. I got the back sacks for $10 each - gave them to the teachers for end of year gifts and they LOVED me!!
It really depends on how important selection is to you on when you should go. Good deals and good selection - go early. Smokin' deals on okay selection - go the last day BUT be there WHEN IT OPENS!