The closing price
The last price of the regular session is called the closing price. It is the number used to say how much a stock rose or fell that day, and it is the starting point for tomorrow's change.
After hours trading
Some trading continues after 4 PM, and again before the market opens. There are fewer buyers and sellers then, so prices can jump around more. Many companies publish their earnings reports right after the close.
Why 5 PM is a good time to catch up
Once the dust settles after the close, the full story of the day is available: how each stock finished, what news drove it and what is coming next. That is why Metric Finance posts your brief at 5 PM Eastern, so the whole day is explained in one read.
Weekends and holidays
The market is closed on weekends and on some public holidays. On those days there is no trading and no new closing price.