A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two solar noons or times the Sun reaches the highest point. The word "day" may also refer to daytime, a time period when the location receives direct and indirect sunlight. On Earth, as a location passes through its day, it experiences morning, noon, afternoon, evening, and night. The effect of a day is vital to many life processes, which is called the circadian rhythm.

A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years. Most calendars' arrangement of dates use either or both the Sun with its four seasons (solar calendar) or the Moon's phasing (lunar calendar). The start of a day is commonly accepted as roughly the time of the middle of the night or midnight, written as 00:00 or 12:00 am in 24- or 12-hour clocks, respectively. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time. Midnight is not the only convention used to determine the start of a new day. Other defining moments have been used throughout history, and some are used even today, such as with the Jewish religious calendar, which counts days from sunset to sunset, so the Jewish Sabbath begins at sundown on Friday. Astronomers also have a convention where their day begins at high noon. This way, all of their observations throughout a single night are recorded as happening on the same day. This method removes ambiguity of a particular observation happening on a calendar day, eliminating the need to further determine which night it happened on. Because when using midnight as the start of day, each calendar day is associated with two separate night periods.

In specific applications, the definition of a day is slightly modified, such as in the SI day (exactly 86,400 seconds) used for computers and standards keeping, local mean time accounting of the Earth's natural fluctuation of a solar day, and stellar day and sidereal day (using the celestial sphere) used for astronomy. In most countries outside of the tropics, daylight saving time is practiced, and each year there will be one 23-hour civil day and one 25-hour civil day. Due to slight variations in the rotation of the Earth, there are rare times when a leap second will get inserted at the end of a UTC day, and so while almost all days have a duration of 86,400 seconds, there are these exceptional cases of a day with 86,401 seconds (in the half-century spanning 1972 through 2022, there have been a total of 27 leap seconds that have been inserted, so roughly once every other year).