Monday is set because: in the case of bad weekends, Mondays generally fall, so the purchase cost is relatively low.

Thursday is chosen because China implements the trading system of T+ 1. If some investors want to use their funds on weekends, they must sell their stocks on Thursday and withdraw their funds on Friday. But when the selling pressure is high, the stock market will fall, and Thursday is called Black Thursday.

Friday is decided because it is a weekend after Friday, and the weekend is uncertain. Many investors are worried about Monday's decline, so they will choose to sell on Friday and hold cash in their hands, so the probability of Friday's decline is also great.

Generally speaking, the fixed investment has not changed much in the long run, mainly by insisting on investment.