I think it's just because thats when the all the 9-5 M-F's have off mainly. I prefer to go on a monday or a tuesday myself because then you can find all the markdowns on sale items that didn't sell over the weekend
The grocery store accross the way is known for their markdowns. As I got some oscar myer turkey salomi for $1.25 for the 1lb pkg & it doesn't exp for 3 weeks!! SCORE!
Though I love going to rummages as well. But since my dh is on the wrong end of the week we haven't gotten to go in some time.
But with gas prices what they are it costs alot more to do rummaging as well.