How long is a year?
How many days in a year?

1Julian Calendar
The old Roman calendar was lunisolar, and thenumber of days in a year might be 354, 355, 377,378, or some other number, as decreed by thepontifex maximus. A basic goal was to observe theSpring Equinox on the same day each year,
although this did not always happen.
Julius Caesar was both consul and pontifex
maximus in 708 AUC (46 BC), which he made 445
days long to correct the date of the spring equinox,
and he instituted a reformed calendar of 365/366days, which took effect on 1 January 709 AUC(45 BC), which was the date of the rst new moonin Rome after the Winter Solstice.
The Julian Calendar, with 29 February as an extra
Leap Day every fourth year, became the calendar ofChristians.

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Gregorian Calendar
Anno Domini (AD) referred to the yearsof the Julian Calendar, which became thecalendar of Christians in Roman times.
The Julian year had an average of 365¼days, but the average tropical year wasactually about 365 days 5 hours 49
minutes 16 seconds, so the date of thespring equinox was slowly changing. By
1582, it was about 10 days off.
A papal bull, Inter gravissimas, of 24February 1582, decreed that the day afterThursday, 4 October 1582 would beFriday, 15 October (not 5 October) 1582.
The new Gregorian Calendar also
improved the insertion of a Leap Day.
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97/400 instead of 1/4
The mean tropical year (for 2000)
is 365.2422 mean solar days long.
The mean northern equinox year(for 2000) is 365.2424 days long.
The Julian Calendar Year is onaverage 365.25 days long.
The Gregorian Calendar Year ison average 365.2425 days long.
John Herschel proposed having
969 leap days every 4000 years(for a 365.24225 day average).
431 December, A.D. 1582
The Papal States, Venice, Spain, Portugal, France, and partsof the Netherlands adopted the Gregorian Calendar in 1582,
but some countries kept the Julian Calendar until the 1900s.
For people using the Gregorian Calendar, 31 December 1582
was a Friday. For people still using the Julian Calendar,
31 December was the Monday, 10 days later.
William III of England arrived at Brixham in England on
5 November 1688 (Julian calendar), after setting sail from theNetherlands on 11 November 1688 (Gregorian calendar).

5365~366 days / year
The Gregorian Calendar reform also helped tostandardize 1 January as the rst day of the year.
In some countries, the year began on a different
day, such as 1 March, 25 March, Easter (different
each year), 1 September, and 25 December.
Whether using Gregorian or Julian, all Europeansagree that each year has either 365 or 366 days.

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the Islamic Calendar
Anno Hegirae (AH) begins with theِ which is whenyear of the Hijra ‫ه ْج َرة‬,Muhammad moved from Mecca toMedina, a month-long migration whichstarted on 26 Safar (17 June) and ended
on 26 Rabi’ I (16 July) in AD 622.
Muslims adopted this numbering ofyears in AH 17, but they kept theirtraditional Arab system of lunar
months.
Thus, AH 1 began on 1 Muharram,
which was Thursday/Friday 15/16 July
AD 622.
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a strictly lunar calendar
12 synodic months is only 12x29.53=354.36 days.
Thus, the Islamic lunar “year” has either 354 or 355 days.

1 Muharram

29~30 days

2 Safar29~30 days

3 Rabi' I29~30 days

4 Rabi' II

29~30 days

5 Jumada I

29~30 days

6 Jumada II

29~30 days

7 Rajab29~30 days

8 Sha'aban

29~30 days

9 Ramadan

29~30 days

10 Shawwal

29~30 days

11 Dhu al-Qi’dah 29~30 days
12 Dhu al-Hijjah 29~30 days
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precession of Islamic holidays
The shorter lunar year results in the precession of Islamicholidays relative to the corresponding Gregorian dates.
Muharram

RamadanA.H.

rst day A.D. ~ last day A.D.

A.H.

rst day A.D. ~ last day A.D.

1429

9 January 2008 ~ 6 February 2008

14291 September 2008 ~ 30 September 2008

1430

29 December 2008 ~ 26 January 20091430

21 August 2009 ~ 19 September 2009

1431

18 December 2009 ~ 15 January 2010

1431

11 August 2010 ~ 9 September 20101432

7 December 2010 ~ 4 January 2011

1432

1 August 2011 ~ 29 August 20111433

26 November 2011 ~ 25 December 20111433

20 July 2012 ~ 18 August 2012

1434

15 November 2012 ~ 13 December 20121434

9 July 2013 ~ 7 August 2013

1435

4 November 2013 ~ 3 December 2013

1435

29 June 2014 ~ 27 July 20141436

25 October 2014 ~ 22 November 2014

1436

18 June 2015 ~ 16 July 2015

143714 October 2015 ~ 12 November 2015

1437

7 June 2016 ~ 5 July 2016

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JapanJapan started using the Gregoriancalendar on 1 January 1873:
明治6年1月1日 (Meiji rokunen
ichigatsu tsuitachi, “the rst dayof the rst month of the 6th yearof the reign of Meiji”)
The preceding day, Gregorian
31 December 1872 was
明治5年12月2日 (Meiji gonen
jūnigatsu futsuka, “the second
day of the twelfth month of the5th year of the reign of Meiji”).

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the 7-day week
The continuous 7-day week has
been used in Judaism since the 6thcentury BC, if not earlier. The
Biblical Sabbath is recorded since
the 9th century BC: The 7th day is
the Sabbath, i.e., the day of rest.
The oldest extant recorded date withthe day of the week also included
was written in AD 60 in Pompei.
The Roman emperor Constantineof cially adopted the 7-day week in
AD 321, and Sunday (dies Solis)
became a holiday.
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other short cycles
The early Roman 8-day nundinal cycle.
The Maya tzolk’in: a 13-day cycle and a 20day cycle, combining to form a 260-daycycle.
The 19th Century French (1793-1805)
décade, a 10-day cycle.

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六曜 rokuyō, 六輝 rokki
先勝 Senshō Good luck before noon, bad luck after noon. Good day for beginnings(in the morning).
友引 Tomobiki

Bad things will happen to your friends. Funerals avoided on thisday. Typically crematoriums are closed this day.
先負 Senbu

Bad luck before noon, good luck after noon.

仏滅 Butsumetsu The most unlucky day. Weddings are avoided. Shinto shrines
sometimes close their of ces. Presumed to be the day Buddha died.
大安 Taian

The luckiest day. Good day for weddings, shop openings, etc.

赤口 Shakkō The hour of the horse (11 am–1 pm) is lucky. The rest is bad luck.

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