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Hello... How come that I have read in many places, that fridays is not a good day to place trades??
Many traders don't like the weekend when the markets are closed. Some news may come out that affects the market.
Some traders close all positions at the end of the trading day for the very same reason.
If you are trading for a living, you have to withdraw money from your account to pay your bills. Obviously you can do so anytime, but many do it on Friday.
The big money, banks, investment firms, close shop after trading hours. That gives lower liquidity. Lower liquidity means more erratic behavior.
The impact of what I wrote depends on what your are trading. A major pair like EURUSD is less effected by those things than many (smaller) stocks. But even huge stocks can be affected. For example if their CEO dies. Or news of fraud comes out. Think Enron. Some traders like hyper aggressive markets. Many don't.
It's also a old saying, from the time everything went manually.