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A **common year** is a calendar year with 365 days, as distinguished from a leap year, which has 366. More generally, a common year is one without intercalation. The Gregorian calendar, (like the earlier Julian calendar), employs both common years and leap years to keep the calendar aligned with the tropical year, which does not contain an exact number of days.

Generally speaking, a **calendar year** begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days. A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. This may be termed a "year's time", but not a "calendar year". To reconcile the calendar year with the astronomical cycle certain years contain extra days.

A **day**, a unit of time, is approximately the period of time during which the Earth completes one rotation around its axis with respect to the Sun. In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86 400 SI seconds and symbolized *d*. In 1967, the second and so the day were redefined by atomic electron transition. A **civil day** is usually 86 400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.

A **leap year** is a calendar year containing one additional day added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting an additional day or month into the year, the drift can be corrected. A year that is not a leap year is called a common year.

The common year of 365 days has 52 weeks and one day, hence a common year always begins and ends on the same day of the week (for example, January 1 and December 31 fell on a Sunday in 2017) and the year following a common year will start on the subsequent day of the week. In common years, February has four weeks, so March will begin on the same day of the week. November will also begin on this day.

A **week** is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar.

In the Gregorian calendar, 303 of every 400 years are common years. By comparison, in the Julian calendar, 300 out of every 400 years are common years, and in the Revised Julian calendar (used by Greece) 682 out of every 900 years are common years.

The **Julian calendar**, proposed by Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on 1 January 45 BC, by edict. 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 **Revised Julian calendar**, also known as the **Milanković calendar**, or, less formally, **new calendar**, is a calendar proposed by the Serbian scientist Milutin Milanković in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church. The Revised Julian calendar temporarily aligns its dates with the Gregorian calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world. The calendar has been adopted by the Orthodox churches of Constantinople, Albania, Alexandria, Antioch, Bulgaria, Cyprus, Greece, Poland, and Romania.

- Common year starting on Sunday
- Common year starting on Monday
- Common year starting on Tuesday
- Common year starting on Wednesday
- Common year starting on Thursday
- Common year starting on Friday
- Common year starting on Saturday

A **common year starting on Sunday** is any non-leap year that begins on Sunday, 1 January, and ends on Sunday, 31 December. Its dominical letter hence is **A**. The most recent year of such kind was 2017 and the next one will be 2023 in the Gregorian calendar, or, likewise, 2018 and 2029 in the obsolete Julian calendar, see below for more. Any common year that starts on Sunday, Monday or Tuesday has two Friday the 13ths. This common year contains two Friday the 13ths in January and October.

A **common year starting on Monday** is any non-leap year that begins on Monday, 1 January, and ends on Monday, 31 December. Its dominical letter hence is **G**. The most recent year of such kind was 2018 and the next one will be 2029 in the Gregorian calendar, or likewise, 2013 and 2019 in the obsolete Julian calendar. The century year, 1900, was also a common year starting on Monday in the Gregorian calendar. See below for more. Any common year that starts on Sunday, Monday or Tuesday has two Friday the 13ths. This common year of this type contains two Friday the 13ths in April and July. Leap years starting on Sunday share this characteristic, but also have another in January.

A **common year starting on Tuesday** is any non-leap year that begins on Tuesday, 1 January, and ends on Tuesday, 31 December. Its dominical letter hence is **F**. The current year, **2019**, is a common year starting on Tuesday in the Gregorian calendar. The last such year was 2013 and the next such year will be 2030, or, likewise, 2014 and 2025 in the obsolete Julian calendar, see below for more. Any common year that starts on Sunday, Monday or Tuesday has two Friday the 13ths. This common year contains two Friday the 13ths in September and December. Leap years starting on Monday share this characteristic. From July of the year that precedes this year until September in this type of year is the longest period that occurs without a Friday the 13th. Leap years starting on Saturday share this characteristic, from August of the common year that precedes it to October in that type of year.

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Images, videos and audio are available under their respective licenses.

**Intercalation** or **embolism** in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.

The **International Fixed Calendar** is a solar calendar proposal for calendar reform designed by Moses B. Cotsworth, who presented it in 1902. It divides the solar year into 13 months of 28 days each. It is therefore a perennial calendar, with every date fixed to the same weekday every year. Though it was never officially adopted in any country, entrepreneur George Eastman adopted it for use in his Eastman Kodak Company, where it was used from 1928 to 1989.

A **common year starting on Friday** is any non-leap year that begins on Friday, 1 January, and ends on Friday, 31 December. Its dominical letter hence is **C**. The most recent year of such kind was 2010 and the next one will be 2021 in the Gregorian calendar, or, likewise, 2011 and 2022 in the obsolete Julian calendar. The century year, 2100, will also be a common year starting on Friday in the Gregorian calendar. See below for more. Any common year that starts on Wednesday, Friday or Saturday has only one Friday the 13th; The only Friday the 13th in this common year occurs in August. Leap years starting on Thursday share this characteristic, but also have another one in February.

**Calendar reform** or **calendrical reform**, is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.

A **common year starting on Wednesday** is any non-leap year that begins on Wednesday, 1 January, and ends on Wednesday, 31 December. Its dominical letter hence is **E**. The most recent year of such kind was 2014, and the next one will be 2025 in the in the Gregorian calendar or, likewise, 2009, 2015, and 2026 in the obsolete Julian calendar. The century year, 1800, was also a common year starting on Wednesday in the Gregorian calendar, see below for more. Any common year that starts on Wednesday, Friday or Saturday has only one Friday the 13th; The only Friday the 13th in this common year occurs in June. Leap years starting on Tuesday share this characteristic.

A **common year starting on Saturday** is any non-leap year that begins on Saturday, 1 January, and ends on Saturday, 31 December. Its dominical letter hence is **B**. The most recent year of such kind was 2011 and the next one will be 2022 in the Gregorian calendar or, likewise, 2017 and 2023 in the obsolete Julian calendar, see below for more. Any common year that starts on Wednesday, Friday or Saturday has only one Friday the 13th; The only Friday the 13th in this common year occurs in May. Leap years starting on Friday share this characteristic.

A **common year starting on Thursday** is any non-leap year that begins on Thursday, 1 January, and ends on Thursday, 31 December. Its dominical letter hence is **D**. The most recent year of such kind was 2015 and the next one will be 2026 in the Gregorian calendar or, likewise, 2010 and 2021 in the obsolete Julian calendar, see below for more. This common year contains the most Friday the 13ths; specifically, the months of February, March, and November. Leap years starting on Sunday share this characteristic.
From February until March in this type of year is also the shortest period that occurs within a Friday the 13th.

The **determination of the day of the week** for any date may be performed with a variety of algorithms. In addition, perpetual calendars require no calculation by the user, and are essentially lookup tables.
A typical application is to calculate the day of the week on which someone was born or a specific event occurred.

A **perpetual calendar** is a calendar valid for many years, usually designed to allow the calculation of the day of the week for a given date in the future.

The **Doomsday rule** is an algorithm of determination of the day of the week for a given date. It provides a perpetual calendar because the Gregorian calendar moves in cycles of 400 years.

The **solar cycle** is a 28-year cycle of the Julian calendar with respect to the week. It occurs because leap years occur every 4 years and there are 7 possible days to start a leap year, making a 28-year sequence.

The **ISO week date** system is effectively a leap week calendar system that is part of the ISO 8601 date and time standard issued by the International Organization for Standardization (ISO) since 1988 and, before that, it was defined in ISO (R) 2015 since 1971. It is used (mainly) in government and business for fiscal years, as well as in timekeeping. This was previously known as "Industrial date coding". The system specifies a *week year* atop the Gregorian calendar by defining a notation for ordinal weeks of the year.

The **Gregorian calendar** is the most widely used civil calendar in the world. It is named after Pope Gregory XIII, who introduced it in October 1582. The calendar spaces leap years to make the average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. The rule for leap years is:

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.

The **Hanke–Henry Permanent Calendar (HHPC)** is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days. It is a modification of a previous proposal, **Common-Civil-Calendar-and-Time** (CCC&T). With the Hanke–Henry Permanent Calendar, every calendar date always falls on the same day of the week.

The **Solar Hijri calendar**, also called the **Solar Hejri calendar** or **Shamsi Hijri calendar**, and abbreviated as **SH**, is the official calendar of Iran and Afghanistan. It begins on the March equinox (Nowruz) as determined by astronomical calculation for the Iran Standard Time meridian and has years of 365 or 366 days.