Through listening to Tom O'Brien I have come to learn about "window dressing". This is the time when money managers pump up winning stocks in order to make their performance look better, although somewhat artificially. If I'm not mistaken, this usually happens at the end of the month.
I just read a fascinating article by the guys at TradingMarkets.com. They did some back testing regarding the end of the month rallies and came to some very interesting conclusions.