To be honest, I've been following the market for quite some time now, and haven't really noticed a big difference in prices during different stages of the season. Your best bet would be days when there are no games, especially Sundays because nobody wants to pay salaries right away. Also hours when there are very few users online (<1000 is what you're looking for).
But there is no guarantee of course. From my experience, you can get a bargain at any time, any day.. you just have to have the patience to look for it...