Every last one of us leaves our mark on history. Most only make a tiny shallow line for our family and friends to notice. A few leave deep grooves that countless other marks align with. Whether you're a believer or not, the historical figure of Jesus Christ is inarguably one of the latter. Much of the western world as we know it was shaped by his life and the stories people tell about him, his life philosophy (as we know it, of course), and a religion others built around him.
One of the widest-used calendars in the world today -- the Gregorian Calendar -- is timed from the birth of this person. It separates history into two large parts: B.C., "before Christ", and A.D., "anno Domini", loosely meaning "year of the Lord". The birth of Jesus should obviously be when we shift between the two.
At least, that's the theory. But it's not that simple. Some historians believe that Jesus was actually born in the year 6 AD, based on the writings of the Roman historian Tacitus. Others argue that Jesus was born earlier, possibly as early as the year 7 BC. To the best of our knowledge, Jesus wasn't born on what we consider to be the 1st anno Domini. With that in mind, though, "the best of our knowledge" on this topic is quite muddy. So roll up your sleeves and let's dive right into it.
Let's get one thing straight first: it wasn't on Christmas day
For starters, although most Christians celebrate the birth of Jesus at Christmas (25th December), we're pretty sure that's not actually when it happened. It was not until the 4th century that the Church of Rome recognized December 25 as the date to celebrate the birth of the Messiah. And it did so knowing well that there were no biblical or historical reasons to place Christ’s birth on that day.
But we can't say when it happened for sure, either. Part of the problem is that Jesus wasn't born famous, so nobody actually bothered to record the exact date. There's also the issue that our current dating system was not even invented yet when it happened, and equating dates between systems is imperfect at best. Factor in the huge spans of time involved here, and accuracy is out of the question.
However, what we do know is that at some point Christianity was an underdog of religions. It had quite an uphill battle gathering new followers in several communities, especially those who were polytheistic. This new, one-god religion was simply very strange to them and the customs they held. People who were better off were also wary of it, as adopting a new religion would often come with a social cost. Not to mention that following teachings which decried slavery and looked down on riches wasn't high on the priority list of people who enjoyed owning slaves and being rich.
In the Roman Empire, the largest single community that Christianity was trying to get into at that time, both of these issues were at work at the same time.
So what Christianity did was a little bit of PR. Christmas today is celebrated very close to the winter solstice. Many ancient peoples aligned their celebrations with significant natural events, such as the solstice. Whether this was intentional or not on their part is a very interesting question, but it's not particularly relevant right now. What is relevant, however, is that by changing dates around a bit, Christian customs would better reflect the pagan ones they were competing against. In other words, it would be more familiar to those it tried to convert. It felt less like a completely new celebration, and more of an updated, reskinned celebration -- and, so, easier to accept.
In the case of Rome, the end of December marked the start of Saturnalia. This was a celebration in honor of their god of the harvest (Saturn) and lasted between the 17th and 23rd, roughly. Symbolically speaking, this was a good celebration to try and associate yourself with, as it was customary for everyone to enjoy freedom during this time, so social norms would be laxer, even discarded altogether. Well, to be more specific, Saturnalia saw an inversion of one's fate.
Slaveowners, for example, would dress, feed, and entertain their slaves like they would a friend. The slaves, in turn, could tell their masters their grievances during this time without fear of reprisal. It was a celebration meant to 'reset your karma', so to speak. Gambling was also allowed on Saturnalia, and carnivals were common. In the grand scheme of things, someone celebrating Christmas would probably stand out far less during Saturnalia than any other time of the year.
This is also probably where we get the custom of gifts during Christmas. Romans exchanged gifts with their friends for Saturnalia, although they were either small figures or gag items, and there most definitely weren't any trees involved.
Of course, none of this actually proves that Christmas was shifted around the calendar to make it more palatable to pagans. But it's very likely that it was, because we're seeing too many coincidences. Further proof that the 25th of December date isn't true to the historical date of Jesus' birth is that the Orthodox Church in the Byzantine half of the Roman Empire set the date of Christmas at January 6th. If one church can change the date, why couldn't another?
The Christmas date origin topic is way broader than I have a taste to get into here, but the Washington Post has a nice breakdown of it here.
Not exactly on time
So we already know the birth date is probably off, although we don't know by how much. The thing to keep in mind here is that the texts which make up books such as the gospel weren't written while Jesus was around, by people who were around him. They were written some time after -- often, a very long time after -- by people working mostly off hearsay. It's not a criticism on their part, it's just the product of a day when writing was still a rare skill, and par for the course of the time.
This material was also heavily curated, edited, tweaked, and cleaned up by (probably) well-meaning but (in my opinion) extremely biased and damaging individuals as Christianity evolved into a mainstream religion. A mainstream religion, after all, needs to have some mainstream-able texts, and working in media, I can assure you that the first copy is never that. Large parts of the initial bible were taken out, and what was left was re-ordered and re-worded to better suit individual agendas. It was an ongoing process, not a single event, as most people who sought power through religion wanted a bible that would fit their narrative better than those of others.
I'm also telling you all that so you'll understand why I don't particularly rely on the biblical texts themselves for answers. They were maintained by people, and people are both fallible and biased. We're also talking about thousands of years here, so there was probably a lot of failing and biased behavior involved. In other words, the biblical texts themselves are not reliable sources if what you're after is to understand what happened and when with accuracy. Not only that, but these are religious texts; they were never intended to preserve chronology, but theology. The dates are not as important as the message, as far as they are concerned.
So when was Jesus born?
While religious texts aren't reliable as direct sources, they do offer useful context. Context which we can then bash against what we know from historical records and archeological digs to hopefully arrive at the truth.
One of the first attempts in this regard was to date the birth of Jesus using the figure of Herod the Great, a puppet ruler appointed by the Romans to oversee order in Judea. In the bible, soon after Herod dies, the new ruler of Judea orders all male infants under two years old in the Bethlehem region (where Jesus was born) to be killed. The good news here is that we have a rough timeline for when Herod died: around 4 B.C. The bad news is that that's not a reliable date by any stretch and that the rest of the story seems to be made up as well. Still, if we take these at face value, Jesus was likely born between the years 6 and 4 B.C.
The story also holds that Jesus' birth was heralded by a star -- the Star of Bethlehem. It has been proposed that this star was actually a slow-moving comet, one that Chinese observers recorded around 5 B.C. This fits well with our previous estimation, which is a plus, but it also basically boils down to "hey these two events fit so they could be the same". This isn't necessarily a wrong conclusion, but it definitely isn't proof.
Reasonable Theology makes a valiant effort of estimating the birth date of Jesus drawing mostly from scripture here (it's a pretty interesting read). I'm not that familiar with everything going on in the bible, so I'll have to take their word for it, but the conclusion they draw from several passages is that Jesus was born sometime between 6 and 5 B.C. This, again, fits with the previous estimation and is a little more reliable as it ties events going on in the story to historical figures such as Emperor Caesar Augustus and Governor Publius Sulpicius Quirinius, which are somewhat well-anchored in history.
It also loosely fits with the Aemilius Secundus inscription, a tablet discovered 300 years ago in Beirut, Lebanon, which tells of a census ordered by Quirinius, the governor of Syria, in 12 B.C., according to biblical scholar Jim Fleming. This census is mentioned in the texts, although different gospels disagree on whether Jesus was born before or after it.
However, there are some grounds to believe that Herod actually died around the year 1 B.C., which would put Jesus's birth around the year 3 B.C.
All things considered, we can estimate with some certainty that Jesus was born between 6 and 4 B.C., and with less certainty that it happened a few years later. But everybody is pretty confident that he -- ironically -- was not born in 'the first year of the lord'.
Since we can't yet know for sure exactly when it happened, this tiny incongruency will have to stick around for a bit longer. With that being said, our calendars are made so practical issues like historical events or yearly tax records can be kept in an organized fashion that future generations will still be able to use, should they need it. Although we think of years as either before or after Christ, they are primarily a chronological tool, not a theological one.