“In the Middle Ages, the study of the measure of time was first viewed as prying too deeply into God’s own affairs – and later thought of as a lowly, mechanical study, unworthy of serious contemplation.”
The calendar we use today is the Gregorian Calendar, first introduced by Pope Gregory XIII in a papal bull (“a formal proclamation issued by the pope usually written in antiquated characters and sealed with a leaden bulla”) in 1582. The reason for the change was to correct an error in the Julian calendar (Caesar’s calendar). The error had apparently been accruing over hundreds of years and, as result the calendar was out of sync with the equinoxes and solstices by an extra day.
Because the calendar didn’t correctly identify the date of Easter, Pope Gregory changed the calendar to be in sync with Easter so that the event would “fall on the first Sunday after the first full moon on or after the Vernal Equinox”.
For this to happen, ten days were removed from the calendar and Pope Gregory mandated that the day following Thursday, October 4, 1582 would be Friday, October 15, 1582, and from that day onward the Gregorian calendar would be used.
According to the modified Gregorian calendar ”Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.”
The Roman Catholic countries in Europe immediately observed the change. Protestant countries, however, refused to change to the new calendar because it was proclaimed by a Catholic Pope.
Here’s a timeline from Wikipedia which reveals a few surprises. It’s also interesting to note that t all Eastern churches continue to use the Julian Easter with the lone exception of the Finnish Orthodox Church, which has adopted the Gregorian Easter.
I hope you find this simple clarification useful for genealogical reasearch.
If you’d like to look at a more detailed explanation click on Gregorian Calendar.
You can also click on Calendars Throughout the Ages to find another interesting explanation.