There are a lot of volatility measures, the most popular being the VIX, but I looked at it a little differently. I looked at the difference for the high and low of the $SPY to see what the range was, and totaled it for the year 2013. A nice small sample.
As you can see above, the day with the biggest swing is Wednesday. Interestingly, it really is hump day, with the action getting larger until Wednesday and then falling off. The numbers about don't normalize for the fact that there are more Monday holidays, but if you take those into account, Monday is still the least volatile.
The data hold up pretty well over prior years of 2012 and 2011 as well. Monday is the least volatile in every year, even correcting for fewer trading days because of holidays. Swing traders may want to consider four day weeks every week. Or at least realize that most of the opportunity to make money is mid-week.