In the past, I've made a big deal about the dangers of making assumptions in genealogical research. Quick refresher: there are three types of assumptions. There are fundamental assumptions, which are the things that -- according to the basic laws of physics and biology -- must be true; there are valid assumptions, which are the things that are generally true, unless proven otherwise; and, finally, there are unsound assumptions, which are those things that could possibly be true, but if we're wrong about them, could mess up our research good and proper.
This time of year, we all focus in on the calendar, which is something that seems to us to be immutable, eternal, marching on inexorably year after year, unchanging and reliable, the fundamental assumption of all fundamental assumptions. We just know that September 3rd will always, always follow September 2nd.
Except for the one year when it didn't.
Let's talk about the calendar and how history and religion made the calendar an unsound assumption for hundreds of years.
In 45 BCE, a revised version of the Roman calendar took effect. Proposed by Julius Caesar, the Julian calendar (named after much deliberation) would be the predominant calendar in Europe, and eventually northern Africa, for the next 1600 years. The previous calendar, known as the calendar of the Roman Republic, had 12 months of with an occasional 13th month of 27 days, Mercedonius, inserted in between February 23rd and 24th in order to make up for the gap between the Roman solar calendar of 355 days and the actual scientific calendar of 365 days, 5 hours, 48 minutes, and 46 seconds. In theory, this insertion of this month was meant to happen every two years, but in practice, it was decided upon by the Pontifex Maximus, who might decide to include it or omit it based upon political expedience. That is to say, whether it would allow his friends to stay in office longer, or whether it would remove his enemies from office sooner, because they could be jerks that way. There were other inconsistencies in this calendar as well; Mercedonius is simply the most glaring example.
Pictured: a jerk.
Julius Caesar fixed a great deal of that, creating a calendar that looks much more familiar to us today: the year in the Julian calendar would have 12 months and be aligned to the sun without any human intervention. The Julian year was 365 days long, and all of our familiar months were set at the same lengths they still hold today. Short term, Caesar actually did quite well with standardizing the calendar, but long term, his calendar had one fatal flaw. He was 11 minutes and 14 seconds off in calculating the length of the year. Why he didn't simply use his iPhone calculator app is an enduring historical mystery. While it seems to be the equivalent of fractions of cents on the dollar, like cents on the dollar it's a compounding error. The more time that elapsed since the implementation of the Julian calendar, the larger and larger that discrepancy of 11 minutes and 14 seconds per year became. The calendar gained an extra day ever 134 years. By 1582 CE, there was a disparity of 10 days in between where the calendar should be and where it was, and had it been left to continue on as it was, that disparity would continue to grow. Distressingly for the Catholic Church (an organization known at this time for levelheadedness and sound priorities), this error also meant that Easter was now increasingly out of alignment with the spring equinox.
Enter Pope Gregory XIII, architect of the Gregorian calendar, which is what we use today. The main difference between the Julian calendar and the Gregorian calendar was that, going forward, leap years, rather than occurring every four years come rain or shine, would not occur in years that ended in two zeros unless the first two numbers of the year were divisible by four. In other words, the years 1700, 1800, and 1900 were not leap years, since 17, 18, and 19 are not divisible by four, but the year 2000 was a leap year, since 20 is divisible by four. In addition to this modification of the leap year scheme, the Gregorian calendar recalculated of the length of the solar year, removing the superfluous 11 minutes, and removed the additional 10 days that had accumulated over the past centuries. The other change of which to be aware is that Pope Gregory standardized the beginning of the year as 1 January. Before that, depending on which country you were in, the beginning of the year might have been 25 December, 1 January, 1 March, or 24/25 March (roughly the spring equinox). Or some communities looked at all the rules, threw up their hands, and said, "Ah hell, it's beer-thirty."
Were this all there was to it, this would seem to be a rather straightforward matter: there was one calendar until 1582 CE, and after that, there was another calendar. However, few things in genealogy are ever that simple. In this case, religious grudges and mistrust rendered the next two centuries a confusing mess to those of us who look back and try to make sense of our ancestors' lives.
The Gregorian calendar having been devised by the Pope, who to Catholics was God's representative on earth, most of Catholic Europe -- Spain, Portugal, Poland, and Italy -- made the switchover to the new calendar immediately, if somewhat grudgingly (much of the populace first viewed it as an attempt by landlords to squeeze an extra 10 days of rent out of them). In those countries, the 4th of October 1582 was followed immediately by the 15th of October. That year, the 5th of October through the 14th of October simply didn't exist. France, some states in the Dutch Republic (including what is today Belgium), and some states in Germany and Switzerland (all Catholic) came onboard shortly thereafter, within a year or two. Hungary also adopted the new calendar relatively early, in 1587 CE.
After that, things got complicated. Protestant Europe, Orthodox Eastern Europe, and Islamic Eastern Europe were all far less willing to incorporate a Catholic innovation into the rhythm of their lives (and East Asia wasn't even a part of the conversation for several hundred more years). Some Protestant rulers feared the new calendar as an insidious Catholic scheme to return them to Catholicism, though the steps in between knowing when Arbor Day is and the rise of an all-powerful papacy in this conspiracy have been lost to the ages. In Anglican England, Queen Elizabeth I thought that dropping the 10 extra days from the calendar wasn't a bad idea, but encountered such fierce resistance from her bishops that she dropped the matter. This marked the last time England would be unnecessarily stubborn on the international stage. Thus, different countries all over Europe were devoted to different, though annoyingly similar, calendars.
Over the centuries, the holdouts slowly came onboard. Denmark, Norway, and the German Protestant states adopted the Gregorian calendar in 1700 CE. Sweden attempted to transition slowly, leading to a calendar that was out of step with both the Julian and Gregorian calendar, and then changed their minds in 1712, reverting to the Julian and adding in a one-time date of 30 February. Children born on this date were to be disappointed for the rest of their lives. Then Sweden changed their minds again in 1753, at which point they and Finland both fully incorporated the Gregorian. Great Britain and all of its colonies, including those in North America, assimilated the new calendar as of 2nd September 1752, which was immediately followed by 14 September, you know, just to keep citizens on their toes.
Some of the more stubborn countries continued to hold out until the 20th century. The Kingdom of Bulgaria transitioned in 1916, the Ottoman Empire in 1917, Russia in 1918, and Greece was the last country in Europe to switch away from the Julian calendar, not making the change until 1923. Since several hundred years had passed since the incorporation of the Gregorian calendar and the 11 minute discrepancy had continued to grow within the Julian calendar, these countries had to drop 13 superfluous days from their calendars, rather than the 10 required of early adaptors. Some of those countries, particularly those adhering to Eastern Orthodox Christianity, continue to this day to use the Julian calendar for religious purposes, as religions have always been known known for their logic and clarity.
Greece: Until recently, a country literally living in the past.
So what does this mean for genealogical purposes? Put simply, when dealing with dates after 1582 CE (and let's face it, most of the dates you'll be working with will be after 1582 CE), you need to have an understanding of when and how that country made the switchover from the Julian calendar to the Gregorian. In some cases, it may even change the story. Let's look at an extreme hypothetical: if your ancestor was recorded as having been born in Yorkshire, England on 2nd September 1752 and baptized on the 14th, you might otherwise think that it was a routine matter. If, on the other hand, you know when and how England made the changeover, you'll realize that this ancestor was, in fact, baptized on the day after birth, which might indicate that he was sickly and not expected to survive. Or, due to the change in when the new year began (in England, for example, until 1752 New Year's Day was 25 March), if your ancestor was recorded in contemporary records as having been born in North Carolina in February 1738, then you need to be aware that said date would actually be 1739 in the Gregorian calendar!
Now go forth and enjoy that you now understand how William Shakespeare and Miguel de Cervantes both died on 23 April 1616... but also died 10 days apart. Happy New Year!