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The leap week calendar is a reformed calendar system with a whole number of weeks every year, and with every year starting on the same weekday.

The ISO week is an example of such a calendar. It is a variation of the Gregorian calendar that is used (mainly) in government and business 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 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, CCC&T as well as the ISO week dates.

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

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.

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

• Leap Week Calendars
• List of ISO leap years - here the leap weeks are called Newton