Since 2000 (N = 1642 trading days), the Dow Jones Industrial Average has lost a little more than 750 points. It turns out, however, that those points are hardly evenly distributed by day of the week.
On Mondays, the Dow has gained a cumulative 1932.63 points;
On Tuesdays, the Dow has gained 265.40 points;
On Wednesdays, the Dow has lost 466.45 points;
On Thursdays, the Dow has gained 1267.72 points;
On Fridays, the Dow has lost 3736.69 points.
In other words, if traders had simply bought at the close on Friday and sold at Thursday's close, instead of losing over 750 points, they would have gained nearly 3000 points.
Since 2006, the Dow is down on the year, but once again is up on a Monday through Thursday basis. Overall, since 2000, we have had 161 up Mondays and 169 down. Conversely, we've had 161 up Mondays and 147 down. (Interestingly, Mondays have not been profitable during 2006).
Could it be that risk-averse traders dump stocks on Friday, not wanting to hold positions over the weekend? If so, they've left an opportunity for those willing to assume risk by buying at the end of the week.