• On one hand you can count off the numbers from 1 to 12 by touching the joints of your fingers with your thumb. There are many good arguments for counting in twelves thather than in tens.
• There are 12 months in a year,
• 12 inches in a foot
• 12 o'clock is midday or midnight.
• There are 12 signs of the Zodiac,
• 12 apostles
• 12 people in a jury.

Why are there 12 Hours in a day?

1. Hour (h or hr) a traditional unit of time, equal to 60 minutes, or 3600 seconds, or 1/24 day. The custom of dividing the daylight into 12 hours goes back at least as far as the Babylonians, who liked to divide units by 12 because groups of 12 are easily divided into halves, thirds, or fourths. Later, when people wanted to express times at night, it was natural to divide the night into 12 hours as well, making 24 hours in the day. Originally an hour was 1/12 of the time between sunrise and sunset, so summer hours were longer than winter hours. Only after the invention of mechanical clocks, around 1300, did hours became equal intervals marked by clocks. The word comes from an ancient Greek word hora which originally meant a season, especially a religious season, and hence a "defined" period of time. In the Christian church hora came to mean one of the services held at seven specific times during the day, thus establishing the word as marking subdivisions of the day.

Why are there 12 items in a Dozen?

1. Dozen (doz or dz) a familiar unit of quantity equal to 12. Division into units of 12 rather than 10 has the advantage that 12 can be evenly divided into halves, thirds, or quarters. For this reason, units of 12 have been common since the earliest civilizations of the Middle East. "Dozen" comes from an old French word dozaine related to the Latin word duodecem, "twelve."

Why are there 12 months in a Year?

1. Month (mo or mon) [1] a unit of time marked by the revolution of the Moon around the Earth. In many traditional societies the appearance of the first tiny crescent moon after the New Moon signaled the start of the month. This start of the month, based on the first appearance of the Moon, is still proclaimed from mosques in Islamic countries. Thus the lunar month is defined as the average interval between two successive moments of New Moon. Astronomers call this period the synodic month. Its length is 29.53059 days.
2. Month (mo or mon) [2] a civil unit of time equal to approximately 1/12 year, but varying from 28 to 31 days. The Sun and the Moon are our traditional time keepers, but they are badly out of step with each other. A solar year equals approximately 12.368 lunar months. The large fraction in this number makes it difficult to design a calendar with a whole number of months in each year. There are at least three solutions to the problem:
   • [i] Use leap months. In the traditional Chinese and Jewish calendars most years have 12 months, but some have a 13th month. In these luni-solar calendars the length of the year varies from 354 to 384 days.
   • [ii] Define 12 synodic lunar months to be a year and don't worry about the length of the year. This is the solution of the Islamic calendar. Since the Islamic year has only 354 or 355 days, its length does not match the cycle of the seasons.
   • [iii] Observe the solar year and let the months be 12 arbitrary periods; don't worry about the Moon. This is the solution adopted by the Romans, who established the civil calendar we use today. In this calendar, all months have 30 or 31 days except the second month, February. February has 28 days in ordinary years and 29 in leap years. See also year [2].

The Earth Orbits the Sun so why do we have 365 Days and not 360?

1. Year (a or y or yr) [2] a traditional unit of time usually equal to 365 or 366 days. We need a whole number of days for the calendar year used in ordinary life. Ancient astronomers knew that the year is approximately 365 days long, and we now know the correct figure is approximately 365.242 days. If we use 365 as the number of days in every calendar year, the extra 0.242 day adds up quickly and causes large errors in predicting the seasons. To solve this problem, the Roman emperor Julius Caesar decreed in 46 BC that the calendar year should have 365 days generally, but that every fourth year should have an extra, or 366th, day. The longer year is called a leap year. In this Julian calendar, four years equal exactly 1461 days, so the average Julian year is exactly 365.25 days.
   • This was a big step toward accuracy in the calendar, but the Julian year is too long by 0.008 day, or a little over 11 minutes. By the time of the Renaissance, these 11-minute errors had accumulated to a total error of about 10 days, so that the spring equinox was occurring near March 11 instead of March 21. In 1582, Pope Gregory XIII decreed that 10 days should be dropped from the calendar: the day after 1582 October 4 was October 15. To reduce future errors, the pope further decreed that years divisible by 100 are not leap years unless they are also divisible by 400. Thus 2000 and 2400 are leap years, but 2100, 2200, and 2300 are not. It took many years, but the Gregorian calendar has now been accepted as the civil calendar in all countries of the world.
   • With the Gregorian adjustment, there are exactly 146 097 days in every 400 years, and the average Gregorian year is exactly 365.2425 days. The Gregorian year is still too long, but by less than half a minute. It will take thousands of years for this error to accumulate to 1 day, so the calendar year and the tropical year are in good enough agreement to last us a long time.