Julian calendar

Old CalendarJulianO.S.OSOld StyleJulian yearJulian calendar reformJJulian reformJulian years
The Julian calendar, proposed by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar.wikipedia
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Gregorian calendar

GregorianN.S.NS
It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII.
The calendar was developed as a correction to the Julian calendar, shortening the average year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.

Adoption of the Gregorian calendar

O.S.Old StyleOS
It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII.
The Gregorian calendar was decreed in 1582 by the papal bull Inter gravissimas by Pope Gregory XIII, to correct a divergence in the canonical date of the [northern] spring equinox from observed reality (due to an error in the Julian system) that affected the calculation of the date of Easter.

Julius Caesar

CaesarGaius Julius CaesarJulius
The Julian calendar, proposed by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar.
After assuming control of government, Caesar began a program of social and governmental reforms, including the creation of the Julian calendar.

January

January is the first month of the year in the Julian and Gregorian calendars and the first of seven months to have a length of 31 days.

February

February 7modern month
February is the second and shortest month of the year in the Julian and Gregorian calendar with 28 days in common years and 29 days in leap years, with the quadrennial 29th day being called the leap day.

March

farchmilitary marchsangatsu
March is the third month of the year in both the Julian and Gregorian calendars.

Computus

date of Eastercalculating Easterbased on the cycles of the moon
The accumulated effect of this difference over some 1600 years since the basis for calculation of the date of Easter was determined at the First Council of Nicea means for example that, from 16 February Julian (1 March Gregorian) 1900 and until 15 February Julian (28 February Gregorian) 2100, Julian is 13 days behind Gregorian.
Because the date is based on a calendar-dependent equinox rather than the astronomical one, there are differences between calculations done according to the Julian calendar and the modern Gregorian calendar.

April

April is the fourth month of the year in the Gregorian calendar, the fifth in the early Julian, the first of four months to have a length of 30 days, and the second of five months to have a length of less than 31 days.

Ianuarius

JanuaryJanuarius, the first month
Ianuarius, fully Mensis Ianuarius (Latin for the "January Month", i.e., "The Month of Janus"), was the first month of the ancient Roman calendar, from which the Julian and Gregorian month of January derived.

May

May 16–18month of May
May is the fifth month of the year in the Julian and Gregorian Calendars and the third of seven months to have a length of 31 days.

June

early JuneMay
June is the sixth month of the year in the Julian and Gregorian calendars, the second of four months to have a length of 30 days, and the third of five months to have a length of less than 31 days.

July

July is the seventh month of the year (between June and August) in the Julian and Gregorian Calendars and the fourth of seven months to have a length of 31 days.

August

month August
August is the eighth month of the year in the Julian and Gregorian calendars, and the fifth of seven months to have a length of 31 days.

Year

myaMamillion years ago
The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days.
The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars; see below.

September

September is the ninth month of the year in the Julian and Gregorian calendars, the third of four months to have a length of 30 days, and the fourth of five months to have a length of less than 31 days.

October

15
October is the tenth month of the year in the Julian and Gregorian Calendars and the sixth of seven months to have a length of 31 days.

November

November is the eleventh and penultimate month of the year in the Julian and Gregorian Calendars, the fourth and last of four months to have a length of 30 days, and the fifth and last of five months to have a length of less than 31 days.

December

No Gender December
December is the twelfth and final month of the year in the Julian and Gregorian Calendars and is the seventh and last of seven months to have a length of 31 days.

Aprilis

Kalendae Aprilis
April had 30 days on calendars of the Roman Republic, with a day added to the month during the reform in the mid-40s BC that produced the Julian calendar.

September (Roman month)

SeptemberMensis SeptemberSeptember 18–22
A day was added to September in the mid-40s BC as part of the Julian calendar reform.

Februarius

February
Februarius was the only month in the pre-Julian calendar to have an even number of days, numbering 28. This was mathematically necessary to permit the year itself to have an odd number of days.

November (Roman month)

NovemberMensis November
A day was added to November during the Julian calendar reform in the mid-40s BC.

Quintilis

Julius
In 45 BC, Julius Caesar instituted a new calendar (the Julian calendar) that corrected astronomical discrepancies in the old.

Iunius (month)

IuniusJuniusJune
Iunius had 29 days until a day was added during the Julian reform of the calendar in the mid-40s BC. The month that followed Iunius was renamed Iulius (July) in honour of Julius Caesar.

Sextilis

AugustusIII Non. Aug.XVI Kal. Sept.
It has sometimes been thought that the month has 31 days because Augustus wanted as many days in his month as in his predecessor's, but Sextilis in fact had 31 days since the reform during Caesar's dictatorship that created the Julian calendar.