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Why do stocks tend to trade at high volumes at the end of (or start) the trading day?
Trading at the start of a session is by far higher than at any other time of the day. This is mostly due to markets incorporating news into the prices of stocks. In other words, there are a lot of factors that can affect a stock, 24 hours a day, but the market trades for only 6.5 hours a day. So, a lot of news accumulates during the time when people cannot trade on that news. Then when markets finally open, people are able to finally trade on that news, and there is a lot of "price discovery" going on between market participants. In the last minutes of trading, volumes increase as well. This can often be attributed to certain kinds of traders closing out their position before the end of the day. For example, if you don't want to take the risk a large price movement at the start of the next day affecting you, you would need to completely close your position.