After 1500 years the Julian calendar was falling far out of step with the sun. Easter was falling too late in the spring. Under pope Gregory XIII the calendar was fixed. That is why it is called the Gregorian calendar.
Years: the years are numbered from the birth of
Christ. Thus “2006″ means 2006 years after the birth of Christ. Years after Christ are called AD for “anno Domini (“in the year of the Lord” in Latin). Years before Christ are called BC for “before Christ”. They run backwards: 13 BC comes after 14 BC.
Week: Every seven days make a week. The days of the week are called:
Jews rest on Saturday.
Months: Each year has twelve months (the number following each is how many days it has):
- January (31)
- February (28)
- March (31)
- April (30)
- May (31)
- June (30)
- July (31)
- August (31)
- September (30)
- October (31)
- November (30)
- December (31)
The first day of spring is March 21st.
Leap day: February sometimes has a leap day, February 29th. The leap day is what keeps the calendar in step with the sun. A calendar is only as good as its rules for leap days.
A year has a leap day:
- If it is a century year, like 1700 or 1600, and can be divided by 400 without anything left over. Example: 1600 and 2000 are leap years, but 1700 and 1900 are not.
- If it is not a century year, then if it can be divided by four without anything left ove. Example: 2004 and 2008 are leap years, but 2005 and 2006 are not.
In effect every fourth year has a leap day, but not always.
The Julian calendar had the second rule but not the first. That is why it was falling out of step with the sun.
But even these rules are not perfect: it will still gain a day every 3333 years. But no one in pope Gregory’s day knew that: the rules were based on the best knowledge of the time: Copernicus’s measurement of the year.
In addition to changing the rules about leap days, the pope also took eleven days out of the calendar: the day after October 4th, 1582 became October 15th, 1582. This was done so spring would begin on March 21st again.
This only happened in Catholic countries. In Protestant and Eastern Orthodox countries the change to the Gregorian calendar took place much later. In the English-speaking world it happened on September 14, 1752: September 2nd was followed by September 14th. By then 12 days had to be taken out!