I have been doing a little research into the peculiarities of certain days of the week. For instance, Monday's tend to have very little economic news released intraday and very few earnings release or reports. You might expect 2008 to have some large intraday turbulence on Mondays, given government reports released over the weekends. But this is not the case. In face, the largest down day on a Monday since 1993 has been approx -4.7% and took place during 2009. The largest up day is 4.3% and also took place during 2009.
Here's a chart displaying the return of $100,000 placed into the SPY at its open on Monday morning and taken out of the SPY at its Monday close. Rather uneventful.