Definition
The Julian calendar set the time it takes the Sun to return to the same point in the seasons at 365 days and 6 hours. To use up the extra 6 hours, it added one leap day every four years. But the real year is a little shorter than that, so over time its dates fell behind the seasons.
The Classroom Clock That Loses 2 Minutes a Day
Picture the clock on the back wall of your classroom losing 2 minutes every day. For a day or two, nobody notices. But after a month, the clock is a whole hour behind.
The Julian calendar was exactly this kind of clock. It set the length of a year at 365 days and 6 hours. Save up those extra 6 hours for four years and you get one full day. So every four years, a day was added to February. We call that a leap year.
The catch is that the real time for the seasons to go around once is a bit shorter. It's about 365 days, 5 hours, and 49 minutes. The seasons finished their lap before the calendar finished its year. So the calendar was falling behind the seasons by about 11 minutes every year.
Eleven minutes is an even slower drift than a clock losing 2 minutes a day. In one person's whole lifetime, it wouldn't add up to even a day, so at first it wasn't a big problem. Scholars only pinned down the drift clearly hundreds of years later, in the Middle Ages.
How 11 Minutes Piled Up Into Ten Days
A clock that loses 2 minutes a day is an hour behind after a month and twelve hours behind after half a year. Even a tiny error becomes easy to see if it never stops adding up.
The calendar's 11 minutes followed the same path. A day has 1,440 minutes, so it takes about 130 repeats of 11 minutes to add up to one day. That means the dates slipped behind the seasons by one day roughly every 130 years.
The Church had fixed the spring equinox at March 21 back in the 4th century (around the Council of Nicaea in 325). Over the thousand-plus years since, the gap grew to nearly ten days. The spring day when daytime and nighttime are equal in length kept showing up earlier and earlier on the calendar. Dates that had been tied to the seasons started to come loose, one by one.
Finally, in 1582, the regions that adopted the new calendar first did what you'd do with a slow clock: they spun it forward. To bring the spring equinox back to around March 21, they erased ten days. The day after October 4 became October 15.
A Fairer Way to Put It
Erasing ten days doesn't mean this calendar was junk. A clock that loses 2 minutes a day is still a great clock if you only need it for a day. This calendar guided dates across Europe for well over a thousand years without much trouble.
The Gregorian calendar that followed is more like a slight tightening of the clock's spring. It kept the rule of adding a day every four years. But years divisible by 100 are not leap years, unless they're also divisible by 400, in which case they are leap years again. That small tweak shrank the leftover error a lot.
Some places still use dates from the older calendar alongside the current one. The two differ by several days, so the date of the same event can be written differently in different sources. That's why the older calendar is often called Old Style and the current one New Style.
It isn't a matter of right or wrong. It's just a difference in how closely you match the dates to the seasons.
Common misconceptions
The Julian calendar had no leap years, which is why its dates drifted from the seasons.
It did have a leap day every four years. The drift came from that extra day being just a little too generous compared with the real year.
People lost ten days of their lives in the year the calendar erased ten days.
Only the names of the dates were skipped, so no actual time disappeared. Still, things counted by the day, like interest or daily wages, had to be adjusted separately.
Where you meet it
When Britain and its colonies switched to the new calendar, eleven days vanished from the month of September.
Some churches celebrate Christmas on a different date because they still follow the dates of the older calendar.
In one sentence
The Julian calendar added a day every four years and set the year at 365 days and 6 hours. That made it fall 11 minutes behind the seasons each year, until ten days finally had to be erased to bring the spring equinox back to around March 21.