Shortest Day Of The Year?
Well, actually not. While December 21st is the day of the winter solstice on the northern hemisphere of our planet, it is actually the longest day for all earthlings when measured from sunrise to sunrise. It's just that on the northern hemisphere most of that time is spent in the dark.
Sometimes I call the day of the week the world's oldest dogma. When we device clocks or calendars, all we do is chop Earths movement through space into little pieces, and give these pieces names. This is done so we can find each other in the fourth dimension; when you and I make an appointment to meet each other in the restaurant on the corner of a certain street in a certain city of a certain country, we'll never meet each other of we do not also agree on a certain time to meet there. It's practical to have a clock and a calendar that we agree upon, but ultimately they are the result of our failed attempts to capture the universe's natural ever moving clockwork in a neat schedule.
You see, the Earth spins around it's own axis in approximately 23.9 hours; this is the time needed for the Earth to spin 360 degrees. But because the planet also spins around the sun in the same counter-clockwise direction (west chasing east), it'll have to turn a tiny fraction more for the same meridian to face the Sun again. Hence the 24 hours instead of 23.9 hours in a day. But this isn't exact at all.
In Europe and most of the western world we've been using the Roman calendar until the Julian calendar was introduced on January 1st in the year 45 BC; this calendar corrected the accumulating error of the calendar being "slow" by one day every four years. A solar year isn't exactly 365 days, but approximately 365.25 days; so every fourth year would henceforth be called a "leap-year" with an extra day added on February 29th. The thing to realize here is that all that changes here is the name of the day of the week. And it appears that the Julian calendar isn't good enough either.
The Catholic Church in 1582 introduced the Gregorian calendar, named after Pope Gregory XIII. The reason was because the date of Easter was tied to the Spring Equinox, the Roman Catholic Church considered the seasonal drift in the date of Easter undesirable. A year isn't 365.25 days either, but 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. So by adding a day every 4 years, we actually add too much, and our calendar does the opposite of lagging behind. To remedy this, Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.
Although this calendar was introduced in 1582 AD, a lot of countries took years to adopt this Gregorian Calendar; the United Kingdom did so in the year 1752, by which time their calendar was 11 days into the future, so on their 1752 calendar we go from Wednesday September 2nd to Thursday September 14th to get rid of those 11 days at once. We still use this calendar today, but even the Gregorian calendar diverges from astronomical observations by one day in 3,030 years...
Earth spins around it's axis, but does so with a slight wobble, which causes the Axial precession, better known as the "precession of the equinoxes", a cycle that takes 25,772 years and was somehow known to the ancient cultures of the planet, like on the famous Maya calendar. And the Earth rotates around the Sun, which rotates in a small local cluster of stars that rotates around the center of the Milky Way Galaxy. The Galaxy rotates with a local cluster of galaxies, which in turn moves through the expanding and accelerating universe... Are you dizzy yet? A date on the calendar is just that; the name we've agreed upon to call our current position in space, wherever that may be :-)