I have noticed over time that the markets seem to be more tradeable earlier to middle of the week.
You could also look at it like this. If Friday's close is compromised by the fact that traders are just getting out for the weekend, than the Monday open would be too, because they are all returning. So Tuesday-Thursday are the real days for accurate data. So there may be something to all that.
Now, of course that observation is through the filter of my own method, so it's just my perspective. Past performance is not indicative of future results and all that.