The Forecast Bar gives you the date, and shows the passage of time from Morning, Afternoon, and Evening. The Forecast Bar does not move during the Night.
During the Day sections, the Forecast Bar shows the weather and temperature forecast.
Changes in weather and temperature help determine how many customers you can expect during the day, and how warm or cool they prefer their cup of lemonade.
Remember, these are just forecasts, and weather predictions are never perfect. You only know the precise weather and temperature for your current time.