Leap week calendar - AbsoluteAstronomy.com

A leap week calendar is a calendar

Calendar

A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms

Calendar reform

A calendar reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar.Most calendars have several rules which could be altered by reform:...

to the civil calendar, but some - such as the ISO week number

ISO week date

The ISO week date system is a leap week calendar system that is part of the ISO 8601 date and time standard. The system is used in government and business for fiscal years, as well as in timekeeping....

calendar - are simply conveniences for specific purposes.

The ISO calendar in question is a variation of the Gregorian calendar

Gregorian calendar

The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

that is used (mainly) in government

Government

Government refers to the legislators, administrators, and arbitrators in the administrative bureaucracy who control a state at a given time, and to the system of government by which they are organized...

and business

Business

A business is an organization engaged in the trade of goods, services, or both to consumers. Businesses are predominant in capitalist economies, where most of them are privately owned and administered to earn profit to increase the wealth of their owners. Businesses may also be not-for-profit...

for fiscal years, 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.

Most leap week calendars take advantage of the fact that 400 Gregorian Calendar

Gregorian calendar

years have exactly 20,871 weeks, so with non-leap years of 52 weeks, this means there are 71 leap weeks every 400 years. These include the Pax Calendar

Pax Calendar

The Pax calendar was invented by James A. Colligan in 1930 as a reform of the Gregorian calendar.Unlike other proposals such as the International Fixed Calendar and the World Calendar, it preserves the 7-day week by intercalating a week to a perpetual year of 52 weeks = 364 days.The year is divided...

, CCC&T

Common-Civil-Calendar-and-Time

The Common-Civil-Calendar-and-Time Calendar 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....

as well as the ISO week date

ISO week date

Advantages

There are no variations between day of week between years for a specific date.
The calendar starts on the same day and week every year.
Unlike the regular calendar, variations of years are limited to a possible addition of a leap week.
There are no fragments of weeks at the end of the year.
Unlike certain proposed calendar reforms such as the World Calendar
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. It is perennial, or perpetual, because it remains the same every year.Each quarter begins...

and International Fixed Calendar
International Fixed Calendar
The International Fixed calendar is a solar calendar proposal for calendar reform designed by Moses B...

, there is no need to modify the week. This avoids opposition from religious groups who object to interruption of the seven-weekday sequence.

Disadvantages

A year with a leap week is at least 7 days longer than a year without a leap 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 Pax Calendar

Pax Calendar

.

People born during a leap week have birthday problems similar to those born on February 29th and furthermore about 1 in 294 days would belong to a leap week rather than about 1 in 1506 days that are February 29th.

In the Gregorian cycle, the fourth quarter accounting statistics may not be consistent over a string of years unless the number of leap weeks are known. For example, a leap week may occur in 6 common years with one leap year or it may occur in 5 common years with two leap years.

Year structures

Calendars with leap week at the end
Week	01	02	03	04		05			06	07	08		09			10		11	12		13		14	15	16		17	18			19		20		21	22			23	24	25	26		27		28		29	30	31			32	33	34	35			36	37	38	39		40	41	42	43		44			45	46	47		48		49	50	51	52	53
Day					\|							\|								\|						\|								\|						\|					\|							\|					\|						\|					\|							\|					\|
13 months	1					2							3								4						5								6						7							8						9						10					11							12						13				*
Bonavian	1								2							3							4								5								6					7									8						9					10								11						12				*
Sym454 Symmetry454 The Symmetry454 Calendar is a proposal for Gregorian calendar reform. It is a perpetual solar calendar that conserves the traditional 7-day week, has symmetrical equal quarters, and starts every month on Monday.-References:...	1					2										3							4					5											6					7						8									9					10					11									12				*
30-31-30	1						2								3								4							5								6						7							8							9						10						11							12					*
Quarter	1																2															3															4
Gregorian	Jan							Feb						Mar								Apr							May								Jun						Jul								Aug							Sep						Oct							Nov						Dec
ISO ISO week date The ISO week date system is a leap week calendar system that is part of the ISO 8601 date and time standard. The system is used in government and business for fiscal years, as well as in timekeeping.... : Mon	Jan					Feb							Mar			Mar							Apr					May											Jun					Jul						Aug									Sep					Oct					Nov			Nov						Dec
ISO: Tue	Jan								Feb							Mar							Apr					May											Jun					Jul						Aug			Aug						Sep					Oct								Nov						Dec
ISO: Wed	Jan								Feb							Mar							Apr					May			May								Jun					Jul									Aug						Sep					Oct								Nov						Dec				Dec
ISO: Thu	Jan								Feb							Mar							Apr								May								Jun					Jul		Jul							Aug						Sep					Oct								Nov						Dec
ISO: Fri	Jan					Feb							Mar								Apr		Apr					May								Jun						Jul								Aug						Sep						Sep		Oct					Nov							Dec
ISO: Sat	Jan					Feb							Mar										Apr					May								Jun								Jul						Aug						Sep			Sep					Oct					Nov							Dec		Dec
ISO: Sun	Jan					Feb							Mar										Apr					May								Jun			Jun					Jul						Aug									Sep					Oct					Nov									Dec

Note that the new years of the calendars shown need not be synchronised.

The years according to ISO week date

ISO week date

applied to months, i.e. a month has as many weeks as it has Thursdays, are shown depending on the weekday of 1 January, shaded weeks belong to the month they are labeled with in regular years and to the other adjoining one in leap years.

External links

Leap Week Calendars
List of ISO leap years - here the leap weeks are called Newton
Hermetic Leap Week Calendar - a leap week calendar with four quarters of three months each, suitable for business purposes

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