Gregorian Calendar: A Unit Study

Gregorian Calendar: A Unit Study
Gregorian Calendar: A Unit Study

On this day, February 24, in 1582 Pope Gregory XIII decreed the Gregorian Calendar the new and official calendar.

Why a New Calendar?

The purpose of the Gregorian calendar was to correct the errors in the previously used Julian calendar. At that time it was thought that the year was 365.25 days long. The Julian calendar had 365 days with an extra day every fourth year. While sufficient for a short amount of time, the Julian calendar gained .0078 days every year on the true solar year. While this number seems reasonably small, it adds up — three days every four centuries. Despite a correction made in AD 325, by the time of Pope Gregory the XIII the calendar was ten days off. This was far too great a number to ignore. Sooner or later annual celebrations would be ridiculously off schedule.

The Corrections Made by the Gregorian Calendar

Pope Gregory XIII proceeded to reform the calendar.

  • First, the extra ten days that had been gained since the date of the correction in AD 325 would have to be removed — so the date would have to go from October 4 on one day to October 15 the next.
  • To correct for the fact that three days were gained every four centuries, Pope Gregory decreed that the “leap years” of the former calendars were to be dropped on dates ending with two zeros, unless the first two digits were able to be evenly divided by four.

These corrections allowed the calendar to stay accurate for an enormous length of time, gaining only one day every ten thousand years, which resulted in its eventual widespread use as the most accurate calendar produced.

Further Investigation

The Gregorian Calendar
Calendar history at The Galileo Project.

Gregorian Calendar
The math behind the calendar.

Definitions of Our Year
Everything about our calendar that you ever wanted to know at WebExhibits online museum.

Leap Year
Facts about leap years — such as when they occur, why they are used, and other interesting facts from Enchanted learning.


Calendar Activity Set
Free download for teaching your children the calendar.

Activity: Using a Calendar

Activity: Using a Calendar
Our helps for teaching your children this skill.

Calendar Activity
Use this interactive calendar from the Franklin Institute to answer the calendar questions below.


“Thirty Days Hath September”
From Rhymes and Jingles.

Our Calendar by George Nichols Packer
Free public domain book that explains the history behind our calendar.

Unit Studies & Lesson Plans

Calendar-Based Activities
Several good suggestions from the BBC.

Telling Time: The Calendar
Lesson plan from The Franklin Institute for teaching young ones about the calendar.

Printables & Notebooking Pages

Jan Brett Interactive Calendar
Make a beautiful calendar for any year!

I Can Make a Calendar
Calendar templates at

Handwriting Calendar Maker
Makes calendars with traceable numbers for those working on handwriting.