With the course of the week, markets usually tend to take an upward trend that peaks on Fridays. This means that it is a good idea to think about shorting stocks on Friday and covering your positions back on Monday when the market gets to lower levels.
Whether because of weekend optimism or because Saturday and Sunday's news hasn't been priced into the market yet, many traders feel that Fridays see stocks and indices priced higher. This could make Friday a good time to sell stocks, hopefully for a slightly better price than they might fetch on Monday.