A leap week calendar is a

calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a phy ...

system with a whole number of

week A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are of ...

s in a

year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the h ...

, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the

civil calendar The civil calendar is the calendar, or possibly one of several calendars, used within a country for civil, official, or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations. Th ...

, in order to achieve a

perennial calendar A perennial calendar is a calendar that applies to any year, keeping the same dates, weekdays and other features. Perennial calendar systems differ from most widely used calendars which are annual calendars. Annual calendars include features parti ...

. Some, however, such as the

ISO week date 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 (last revised in 2019) and, before that, it was ...

calendar, are simply conveniences for specific purposes. The ISO calendar in question is a variation of the

Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years d ...

that is used (mainly) in government and business for

fiscal year A fiscal year (or financial year, or sometimes budget year) is used in government accounting, which varies between countries, and for budget purposes. It is also used for financial reporting by businesses and other organizations. Laws in many ...

s, as well as in timekeeping. In this system a year (ISO year) has 52 or 53 full weeks (364 or 371 days). Leap week calendars vary on whether the concept of month is preserved and whether the month (if preserved) has a whole number of weeks. The

Pax Calendar The Pax calendar was invented by James A. Colligan, SJ in 1930 as a perennializing reform of the annualized Gregorian calendar. Design The common year is divided into 13 months of 28 days each, whose names are the same as in the Gregorian cal ...

and

Hanke–Henry Permanent Calendar The Hanke–Henry Permanent Calendar (HHPC) is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars that maintain synchronization with the solar year by intercalating entire weeks rather than single days. ...

preserve or modify the Gregorian month structure. The ISO week date is an example of a leap week calendar that eliminate the month. A leap week calendar can take advantage of the 400-year cycle of the Gregorian calendar, as it has exactly 20,871 weeks: with 329 common years of 52 weeks plus 71 leap years of 53 weeks, a leap week calendar would synchronize with the Gregorian every 400 years since

Advantages

* The calendar starts on the same day of the week every year. * There are no fragments of weeks at the beginning or end of the year. * Unlike the

Gregorian Calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years d ...

, variations of years are limited to the possible addition of a leap week. * Unlike certain proposed calendar reforms such as the

World Calendar The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930. Features The World Calendar is a 12-month, perennial calendar with equal quarters. Each quarter begins on a Sunday ...

and

International Fixed Calendar The International Fixed Calendar (also known as the IFC, Cotsworth plan, the Cotsworth calendar and the Eastman plan) is a proposed calendar reform designed by Moses B. Cotsworth, first presented in 1902. The solar calendar divides the year into ...

, there are no exceptions to the 7 day cycle of the week. This avoids opposition from religious groups who object to the interruption of the weekday sequence.

Disadvantages

* Although the calendar starts on the same day of the week every year, not all countries observe the same day as the start of their

. This will therefore present an issue if a leap week calendar is intended for use in multiple countries. * A year with an intercalary/leap week is 7 days longer than a year without an intercalary week. Consequently, the equinoxes and solstices must vary over 7 days, i.e. ±3 of the average date, or even more, such as 19 days in the

. * While persons born during the added intercalary week lose their real birthday in common years, similarly to those born on 29 February in the Gregorian calendar, approximately 1 in 294 days would belong to an intercalary week, compared to the approximately 1 in 1506 days that occur on 29 February. * Leap year rules are more complicated than the Gregorian, as there is no simple approximation like one in four years since leap years are not at fixed intervals: see

and and . * Quarterly accounting statistics will not be consistent over multiple years due to the yearly quarter containing the intercalary week having 14 weeks instead of the usual 13. This issue could arguably be minimised by placing the intercalary week at the end of the year.

References

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External links

Hermetic Leap Week Calendar
– a leap week calendar with four quarters of three months each, suitable for business purposes