A leap week calendar is a calendar system with a whole number of weeks in a year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, in order to achieve a perennial calendar. Some, however, such as the ISO week date calendar, are simply conveniences for specific purposes.

The ISO calendar in question 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. The Pax Calendar and Hanke–Henry Permanent Calendar 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 (329 × 52 + 71 × 53 = 20,871).