Friday. Institutional investors like to take some profits, just before the weekend, as a way to hedge against any serious negative news.
This is widely known, so some use this modest selling to their advantage, by shorting certain favorites and then selling them on margin.
If you are quick and lucky, you can buy these stocks at a small discount. Because they sold shares on margin, these investors need to replace the shares by buying the same amount. Usually in the last minutes of the day.