Leap week calendar
Encyclopedia
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
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...

 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
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....

s.

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
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...

.

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
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....

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

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