Day view displays a single day of your calendar.
To view to earlier or later times in the current day, scroll up or down.
To scroll backward or forward one day at a time, click the left or right arrow button, swipe left or right with two fingers on a trackpad, swipe left or right with one finger with a Magic Mouse, or press ⌘-Left Arrow or ⌘-Right Arrow.
You can change Day view to show any range from 4–24 hours by going to BusyCal > Preferences > General and choosing the desired number of hours from the “Day and Week views show” pop-up menu. Or hold down the option key while swiping up/down to zoom in/out. You can also change various display styles in Appearance Preferences.