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.

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.

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.

- 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

1703 (MDCCIII)

was a common year starting on Monday of the Gregorian calendar and a common year starting on Friday of the Julian calendar, the 1703rd year of the Common Era (CE) and Anno Domini (AD) designations, the 703rd year of the 2nd millennium, the 3rd year of the 18th century, and the 4th year of the 1700s decade. As of the start of 1703, the Gregorian calendar was

11 days ahead of the Julian calendar, which remained in localized use until 1923. In the Swedish calendar it was a common year starting on Thursday, one day ahead of the Julian and ten days behind the Gregorian calendar.

17061706 (MDCCVI)

was a common year starting on Friday of the Gregorian calendar and a common year starting on Tuesday of the Julian calendar, the 1706th year of the Common Era (CE) and Anno Domini (AD) designations, the 706th year of the 2nd millennium, the 6th year of the 18th century, and the 7th year of the 1700s decade. As of the start of 1706, the Gregorian calendar was

11 days ahead of the Julian calendar, which remained in localized use until 1923. In the Swedish calendar it was a common year starting on Monday, one day ahead of the Julian and ten days behind the Gregorian calendar.

17071707 (MDCCVII)

was a common year starting on Saturday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1707th year of the Common Era (CE) and Anno Domini (AD) designations, the 707th year of the 2nd millennium, the 7th year of the 18th century, and the 8th year of the 1700s decade. As of the start of 1707, the Gregorian calendar was

11 days ahead of the Julian calendar, which remained in localized use until 1923. In the Swedish calendar it was a common year starting on Tuesday, one day ahead of the Julian and ten days behind the Gregorian calendar.

17091709 (MDCCIX)

was a common year starting on Tuesday of the Gregorian calendar and a common year starting on Saturday of the Julian calendar, the 1709th year of the Common Era (CE) and Anno Domini (AD) designations, the 709th year of the 2nd millennium, the 9th year of the 18th century, and the 10th and last year of the 1700s decade. As of the start of 1709, the Gregorian calendar was

11 days ahead of the Julian calendar, which remained in localized use until 1923. In the Swedish calendar it was a common year starting on Friday, one day ahead of the Julian and ten days behind the Gregorian calendar.

17101710 (MDCCX)

was a common year starting on Wednesday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1710th year of the Common Era (CE) and Anno Domini (AD) designations, the 710th year of the 2nd millennium, the 10th year of the 18th century, and the 1st year of the 1710s decade. As of the start of 1710, the Gregorian calendar was

11 days ahead of the Julian calendar, which remained in localized use until 1923. In the Swedish calendar it was a common year starting on Saturday, one day ahead of the Julian and ten days behind the Gregorian calendar.

17911791 (MDCCXCI)

was a common year starting on Saturday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1791st year of the Common Era (CE) and Anno Domini (AD) designations, the 791st year of the 2nd millennium, the 91st year of the 18th century, and the 2nd year of the 1790s decade. As of the start of 1791, the Gregorian calendar was

11 days ahead of the Julian calendar, which remained in localized use until 1923.

18691869 (MDCCCLXIX)

was a common year starting on Friday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1869th year of the Common Era (CE) and Anno Domini (AD) designations, the 869th year of the 2nd millennium, the 69th year of the 19th century, and the 10th and last year of the 1860s decade. As of the start of 1869, the Gregorian calendar was

12 days ahead of the Julian calendar, which remained in localized use until 1923.

18751875 (MDCCCLXXV)

was a common year starting on Friday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1875th year of the Common Era (CE) and Anno Domini (AD) designations, the 875th year of the 2nd millennium, the 75th year of the 19th century, and the 6th year of the 1870s decade. As of the start of 1875, the Gregorian calendar was

12 days ahead of the Julian calendar, which remained in localized use until 1923.

18791879 (MDCCCLXXIX)

was a common year starting on Wednesday of the Gregorian calendar and a common year starting on Monday of the Julian calendar, the 1879th year of the Common Era (CE) and Anno Domini (AD) designations, the 879th year of the 2nd millennium, the 79th year of the 19th century, and the 10th and last year of the 1870s decade. As of the start of 1879, the Gregorian calendar was

12 days ahead of the Julian calendar, which remained in localized use until 1923.

1879 (MDCCCLXXIX)

was a common year starting on Wednesday of the Gregorian calendar and a common year starting on Monday of the Julian calendar, the 1879th year of the Common Era (CE) and Anno Domini (AD) designations, the 879th year of the 2nd millennium, the 79th year of the 19th century, and the 10th and last year of the 1870s decade. As of the start of 1879, the Gregorian calendar was

12 days ahead of the Julian calendar, which remained in localized use until 1923.

18811881 (MDCCCLXXXI)

was a common year starting on Saturday of the Gregorian calendar and a common year starting on Thursday of the Julian calendar, the 1881st year of the Common Era (CE) and Anno Domini (AD) designations, the 881st year of the 2nd millennium, the 81st year of the 19th century, and the 2nd year of the 1880s decade. As of the start of 1881, the Gregorian calendar was

12 days ahead of the Julian calendar, which remained in localized use until 1923.

20192019 (MMXIX)

is the current year, and is a common year starting on Tuesday of the Gregorian calendar, the 2019th year of the Common Era (CE) and Anno Domini (AD) designations, the 19th year of the 3rd millennium, the 19th year of the 21st century, and the 10th and last year of the 2010s decade.

2019 has been designated as International Year of the Periodic Table of Chemical Elements by the United Nations General Assembly given that it coincides with the 150th anniversary of its creation by Dmitri Mendeleev in 1869.

Common year starting on FridayA common year starting on Friday is any non-leap year (i.e. a year with 365 days) 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.

Common year starting on MondayA common year starting on Monday is any non-leap year (i.e., a year with 365 days) 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 (the current year) 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.

Common year starting on SaturdayA common year starting on Saturday is any non-leap year (i.e. a year with 365 days) 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.

Common year starting on SundayA common year starting on Sunday is any non-leap year (i.e. a year with 365 days) 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.

Common year starting on ThursdayA common year starting on Thursday is any non-leap year (i.e. a year with 365 days) 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 (one month) that occurs within a Friday the 13th.

Common year starting on TuesdayA common year starting on Tuesday is any non-leap year (i.e. a year with 365 days) 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 (14 months) 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.

Common year starting on WednesdayA common year starting on Wednesday is any non-leap year (i.e. a year with 365 days) 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, 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.

Leap yearA leap year (also known as an intercalary year or bissextile year) is a calendar year containing one additional day (or, in the case of lunisolar calendars, a month) 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 (also called intercalating) 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.

For example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28. These extra days occur in years which are multiples of four (with the exception of centennial years not divisible by 400). Similarly, in the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. In the Bahá'í Calendar, a leap day is added when needed to ensure that the following year begins on the vernal equinox.

The name "leap year" probably comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, the day of the week in the 12 months following the leap day (from March 1 through February 28 of the following year) will advance two days due to the extra day (thus "leaping over" one of the days in the week). For example, Christmas Day (December 25) fell on a Sunday in 2016, and Monday in 2017, then it fell on Tuesday in 2018, and will fall on Wednesday in 2019 but then "leaps" over Thursday to fall on a Friday in 2020.

The length of a day is also occasionally changed by the insertion of leap seconds into Coordinated Universal Time (UTC), owing to the variability of Earth's rotational period. Unlike leap days, leap seconds are not introduced on a regular schedule, since the variability in the length of the day is not entirely predictable.

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