The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems.
A day is defined as time required for a celestial body to turn once on its axis – especially the period of the Earth’s rotation.
A sidereal day is the time required for the Earth to rotate once relative to the background of the stars — i.e., the time between two observed passages of a star over the same meridian of longitude. It is approximately 86,164 seconds or 23 h 56 min.
A solar day is the time between two successive transits of the Sun over the same meridian. Because the orbital motion of the Earth makes the Sun seem to move slightly eastward each day relative to the stars, a solar day is about four minutes longer than a sidereal day. It is approximately 24 hours 3 minutes 56.555 seconds.
A civil day is usually 24 hours, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.
The unit of measurement “day” (symbol d) is defined as 86,400 SI seconds. The second is designated the SI base unit of time.
In law the word day, unless qualified, means the 24 hours between midnight and midnight, rather than the daylight hours between sunrise and sunset.
The term “day” comes from the Old English dæg, with its cognates such as dagur in Icelandic, tag in German, and dag in Norwegian, Danish, Swedish and Dutch – all stemming from a Proto-Germanic root *dagaz.
In the Hebrew Bible, Genesis 1:5 defines a day in terms of “evening” and “morning” before recounting the creation of a sun to illuminate it: “And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.” Common convention among the ancient Romans, ancient Chinese and in modern times is for the civil day to begin at midnight, i.e. 00:00, and to last a full 24 hours until 24:00 (i.e. 00:00 of the next day). In ancient Egypt the day was reckoned from sunrise to sunrise. The Jewish day begins at either sunset or nightfall (when three second-magnitude stars appear)
Medieval Europe also followed this tradition, known as Florentine reckoning: in this system, a reference like “two hours into the day” meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning.
A holiday is a day set aside by custom or by law on which normal activities, especially business or work including school, are suspended or reduced. Generally, holidays are intended to allow individuals to celebrate or commemorate an event or tradition of cultural or religious significance. Holidays may be designated by governments, religious institutions, or other groups or organizations. The degree to which normal activities are reduced by a holiday may depend on local laws, customs, the type of job held or personal choices.
Christmas Day is a public holiday in many countries, is celebrated religiously by a majority of Christians, as well as culturally by many non-Christians, and forms an integral part of the holiday season organized
around it. “Christmas” is a shortened form of “Christ’s mass”. The word is recorded as Crīstesmæsse in 1038 and Cristes-messe in 1131. Crīst (genitive Crīstes) is from Greek Khrīstos (Χριστός), a translation of Hebrew Māšîaḥ (מָשִׁיחַ), “Messiah”, meaning “anointed” – and mæsse is from Latin missa, the celebration of the Eucharist.
The summer solstice, which is the longest day of the year and takes place in the northern hemisphere when the Sun is directly over the Tropic of Cancer. For the northern half of Earth (the Northern Hemisphere), the summer solstice occurs annually on between June 20 and June 22. (For the Southern Hemisphere, the summer solstice occurs between December 20 and December 23.)
The winter solstice is the day with the fewest hours of sunlight in the whole year, making it the shortest day of the year. For the northern half of Earth (the Northern Hemisphere), the winter solstice occurs annually between December 20 and December 23 – for the Southern Hemisphere, the winter solstice occurs between June 20 and June 22.
The Midnight Sun also known as polar day refers to an extended period of daytime that lasts 24 hours or more. The phenomenon occurs in or near the Arctic and Antarctic regions during summer.
As of 2021, day is the 205th most common word in US English, and the 210th most common in UK English.
“The two most important days in your life are the day you are born and the day you find out why.” – Mark Twain.
For most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day’s boundaries.
Besides a sidereal day on Earth, other bodies in the Solar System have day times, the durations (daylength in hours) of these being :
Ceres: 9 – 9.1 hours
Earth’s Moon: 708.7