Calendars and time
Intercalary Day
Direct answer
Intercalary Day: An Intercalary Day is an added calendar day inserted outside the ordinary annual pattern to improve alignment with the seasonal year.
An Intercalary Day is an extra day added by a calendar correction rule.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Intercalation, Intercalary Month, Calendar, Julian Date
An Intercalary Day is an extra day added by a calendar correction rule.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Intercalary Day
Roman, Egyptian-derived, Gregorian, and other calendars have placed supplementary days according to different structures.
Why Intercalary Day matters
The added day absorbs fractional-year accumulation that cannot be represented by a calendar made only of whole days.
How to read Intercalary Day
Intercalary Day speaks to leap-day history and for calendars that insert a supplementary day in places other than February.
Intercalary Day in practice: an example
February 29 is the Gregorian calendar best-known Intercalary Day.
Where readings of Intercalary Day go wrong
A common error is to call every festival day outside a normal month intercalary unless it performs a calendar-alignment function.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Introduction to Calendars - Official overview of calendar systems, leap rules, eras, and historical calendar reform.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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More in Calendars & Timekeeping
- Leap Second
- Modified Julian Date
- Sidereal Day
- Tropical Month
- Anomalistic Year
- Calendar Month
- Dominical Letter
- Equation of Time
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Common questions
What is an Intercalary Day?
An Intercalary Day is an added calendar day inserted outside the ordinary annual pattern to improve alignment with the seasonal year.
What do people get wrong about an Intercalary Day?
A common error is to call every festival day outside a normal month intercalary unless it performs a calendar-alignment function.
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