April is the fourth month of the year in the Julian and Gregorian calendars, and one of four months with a length of 30 days.

August ( ) is the eighth month of the year in the Julian and Gregorian Calendars and one of seven months with a length of 31 days.

Astronomers use the Julian calendar for years before 1582, including this year 0, and the Gregorian calendar for years after 1582 as exemplified by Jacques Cassini ( 1740 ), Simon Newcomb ( 1898 ) and Fred Espenak ( 2007 ).

* year, month, and day – most systems, including the Gregorian calendar ( and its very similar predecessor, the Julian calendar ), the Islamic calendar, and the Hebrew calendar

Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar or the astronomical year numbering system.

Calculating the calendar of a previous year ( for the Gregorian calendar taking account of the week ) is a relatively easy matter when Easter Sunday is not included on the calendar.

The astronomer's mean tropical year which is averaged over equinoxes and solstices is currently 365. 24219 days, slightly shorter than the average length of the year in most calendars, but the astronomer's value changes over time, so William Herschel's suggested correction to the Gregorian calendar may become unnecessary by the year 4000.

However, in order to regard 2000 as the first year of the 21st century according to the astronomical year numbering, the astronomical year 0 has to correspond to the Gregorian year 1 BC.

The ISO does present a standard for identifying weeks, but as it does not match up with Gregorian calendar ( the beginning and ending days of a given year do not match up ), this standard is somewhat more problematic than the other standards for dates.

Each year, the Eastern Orthodox Church celebrates the anniversary of the consecration of the Church of the Resurrection ( Holy Sepulchre ) on September 13 ( for those churches which follow the traditional Julian calendar, September 13 currently falls on September 26 of the modern Gregorian calendar ).

December ( ) is the 12th and last month of the year in the Julian and Gregorian Calendars and one of seven months with the length of 31 days.

In addition, without changes to either calendar, the frequency of monthly divergence between the two festivals will increase over time as a result of the differences in the implicit solar years: the implicit mean solar year of the Hebrew calendar is 365. 2468 days while that of the Gregorian calendar is 365. 2425 days.

Because the Julian calendar's implicit solar year has drifted further over the centuries than those of the Gregorian or Hebrew calendars, Julian Easter is a lunation later than Gregorian Easter in five years out of nineteen, namely years 3, 8, 11, 14, and 19 of the Christian cycle.

February ( or or ) is the second month of the year in the Julian and Gregorian calendars.

February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of the year.

A majority of Orthodox and Hasidic Jews believe that the Hebrew calendar dates back to the time of creation ; the year 2009-2010 ( the Hebrew New Year begins during September or October ) of the Gregorian calendar corresponds to the Hebrew year 5770.

The Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar.

Note: Yom Kippur begins at sundown of the previous day in the Gregorian calendar ( e. g. 25 September 2012 ).

The Iranian year usually begins within a day of 21 March of the Gregorian calendar.

* The Dies irae begins at bar 127, the motif derived from a Gregorian chant.

The 23rd century is the century of the Christian Era or Common Era which, in the Gregorian calendar, begins on January 1, 2201 and ends on December 31, 2300.

In the Eastern Orthodox Church, The Great Feast of the Nativity of our Lord begins on the Eve of 25 December ( for those Orthodox churches which follow the Julian calendar, 25 December falls on 7 January of the modern Gregorian Calendar ).

The Salivahana era begins its count of years from the year corresponding to 78 CE of the Gregorian calendar.

Within each 100-year block, the cyclic nature of the Gregorian calendar proceeds in exactly the same fashion as its Julian predecessor: A common year begins and ends on the same day of the week, so the following year will begin on the next successive day of the week.

The Discordian year is aligned with the Gregorian calendar and begins on January 1, thus Chaos 1, YOLD is January 1,

In three parts, it begins with a study of Boethius, who wrote extensively on music in the sixth century ; it traces the use of the musical modes in plainsong ( e. g. Gregorian chant ) and monophony ; and it closes with an extended study of the use of modes in polyphony.

The second three-part movement in the album, " Back to the Rivers of Belief ", begins slowly with John Williams ' five-toned notes from Steven Spielberg's movie, Close Encounters of the Third Kind, which leads to mellow Gregorian chants in the first part of the movement, " Way to Eternity ".

The main drawback of this rule is that the new year varies 17 days relative to the Gregorian new year ( because year 1965 begins 11 days earlier than Gregorian 1965 and year 2036 begins 6 days later than Gregorian 2036 ).

Again in Western India the same era begins with the first day after the new moon in the month of Kartika which usually falls in October and November in the Gregorian calendar.

In the Gregorian calendar, it usually begins around 20 January and ends around 4 February.

Since in Orthodox liturgical practice the day begins at sunset, the feast is celebrated on 19 July ( the Russian Orthodox Church follows the traditional Julian Calendar, so 19 July corresponds to 1 August on the modern Gregorian Calendar ).

The second section, detailing the Gregorian mission of Augustine of Canterbury was framed on the anonymous Life of Gregory the Great written at Whitby.

The last section, detailing events after the Gregorian mission, Goffart feels were modelled on Stephen of Ripon's Life of Wilfrid.

Due to the change from the Julian calendar to the Gregorian calendar, Bealltainn in Scotland was commonly celebrated on 15 May while in Ireland " Old Beltane " or " Old May " began about the night of 11 May.

For example, the Gregorian calendar has no inherent dependence on the seven-day week, but in Western society the two are used together, and calendar tools indicate both the Gregorian date and the day of week.

It is used to date events in most of the Muslim countries ( concurrently with the Gregorian calendar ), and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals.

This set of 12 months can start and end at any point on the Gregorian calendar.

It is common to display the Gregorian calendar in separate monthly grids of seven columns ( from Monday to Sunday, or Sunday to Saturday depending on which day is considered to start the week-this varies according to country ) and five to six rows ( or rarely, four rows when the month of February contains 28 days beginning on the first day of the week ), with the day of the month numbered in each cell, beginning with 1.

According to the Gregorian calendar, the 1st century AD / CE started on January 1, 1 and ended on December 31, 100.

The terms " Common Era ", " Anno Domini ", " Before the Common Era " and " Before Christ " in contemporary English can be applied to dates that rely on either the Julian calendar or the Gregorian calendar.

Hanukkah is observed for eight nights and days, starting on the 25th day of Kislev according to the Hebrew calendar, which may occur at any time from late November to late December in the Gregorian calendar.

Coptic Orthodox Christians celebrate Christmas on 7 January ( Gregorian Calendar ), which coincides with 25 December according to the Julian Calendar.

Eastern Christianity bases its calculations on the Julian calendar whose 21 March corresponds, during the 21st century, to 3 April in the Gregorian calendar, in which the celebration of Easter therefore varies between 4 April and 8 May.

Easter and the holidays that are related to it are moveable feasts, in that they do not fall on a fixed date in the Gregorian or Julian calendars ( both of which follow the cycle of the sun and the seasons ).

In Western Christianity, using the Gregorian calendar, Easter always falls on a Sunday between 22 March and 25 April, inclusively.

Easter therefore varies between 4 April and 8 May on the Gregorian calendar ( the Julian calendar is no longer used as the civil calendar of the countries where Eastern Christian traditions predominate ).

Since 1582, when the Catholic Church adopted the Gregorian calendar while the Eastern Orthodox and most Oriental Orthodox Churches retained the Julian calendar, the date on which Easter is celebrated has again differed.

Accordingly, Gregorian Easter can fall on 35 possible dates — between 22 March and 25 April inclusive.