The middle of the day tends to be the calmest and most stable period of the trading day. During this time, people are waiting for further news to be announced. Because most of the day’s news releases have already been factored into stock prices, many are watching to see where the market may be heading for the remainder of the day.

It's generally impossible to time the market; knowing when the market will be up or when it will be down. Plenty of research, such as knowing the financials of the company you are interested in and the condition of the overall economic environment can help you make the right timing decisions.
In the United States, Fridays on the eve of three-day weekends tend to be especially good. Due to generally positive feelings before a long holiday weekend, the stock markets tend to rise ahead of these observed holidaysBest Day of the Week to Sell Stocks
If Monday may be the best day of the week to buy stocks, then Thursday or early Friday may be the best day to sell stock—before prices dip. If you’re interested in short selling, then Friday may be the best day to take a short position (if stocks are priced higher on Friday), and Monday would be the best day to cover your shortKey Takeaways
Traders often have set holding periods to time when to enter and exit their trades.
Day trading, as the name implies, has the shortest time frame, with trades broken down into hours, minutes, and even seconds. The time of day when a trade is made can be an important factor to consider.
The closest thing to a hard-and-fast rule is that the first hour and last hour of a trading day are the busiest, offering the most opportunities, while the middle of the day tends to be the calmest and most stable period of most trading days.
Some traders believe that certain days offer systematically better returns than others, but over the long run, there is little evidence for such a market-wide effect..