In many traditional and religious contexts, a “day” is defined as the time from sunrise to sunset. That means:

• Day begins when the sun first appears above the horizon in the morning (sunrise).
• Day ends when the sun dips below the horizon in the evening (sunset).

How long that daylight period lasts depends on where you are and the season. Near the equator, day and night are roughly equal year-round. In higher latitudes, days are longer in summer and shorter in winter. So “sunrise to sunset” gives you the daylight portion of a 24-hour day, while the night is the interval from sunset to the next sunrise.

In some traditions, a day is counted from dawn (the first light before sunrise) or from one sunrise to the next; in others, civil time uses midnight to midnight. The exact definition can vary by culture and purpose, but sunrise-to-sunset is a common, straightforward way to mark a day by daylight.