The Curious History of the Gregorian Calendar

Updated March 18, 2021 | Ben Snowden

Eleven days that never were

September 2, 1752, was a great day in the history of sleep.

That Wednesday evening, millions of British subjects in England and the colonies went peacefully to sleep and did not wake up until twelve days later. Behind this feat of narcoleptic prowess was not some revolutionary hypnotic technique or miraculous pharmaceutical discovered in the West Indies. It was, rather, the British Calendar Act of 1751, which declared the day after Wednesday the second to be Thursday the fourteenth.

Prior to that cataleptic September evening, the official British calendar differed from that of continental Europe by eleven days—that is, September 2 in London was September 13 in Paris, Lisbon, and Berlin. The discrepancy had sprung from Britain's continued use of the Julian calendar, which had been the official calendar of Europe since its invention by Julius Caesar (after whom it was named) in 45 B.C.

Caesar's calendar, which consisted of eleven months of 30 or 31 days and a 28-day February (extended to 29 days every fourth year), was actually quite accurate: it erred from the real solar calendar by only 11½ minutes a year. After centuries, though, even a small inaccuracy like this adds up. By the sixteenth century, it had put the Julian calendar behind the solar one by 10 days.

In 1582, Pope Gregory XIII ordered the advancement of the calendar by 10 days and introduced a new corrective device to curb further error: century years such as 1700 or 1800 would no longer be counted as leap years, unless they were (like 1600 or 2000) divisible by 400.

If somewhat inelegant, this system is undeniably effective, and is still in official use in the United States. The Gregorian calendar year differs from the solar year by only 26 seconds—accurate enough for most mortals, since this only adds up to one day's difference every 3,323 years.

Despite the prudence of Pope Gregory's correction, many Protestant countries, including England, ignored the papal bull. Germany and the Netherlands agreed to adopt the Gregorian calendar in 1698; Russia only accepted it after the revolution of 1918, and Greece waited until 1923 to follow suit. And currently many Orthodox churches still follow the Julian calendar, which now lags 13 days behind the Gregorian.

Why So Difficult?

Since their invention, calendars have been used to reckon time in advance, and to fix the occurrence of events like harvests or religious festivals. Ancient peoples tied their calendars to whatever recurring natural phenomena they could most easily observe. In areas with pronounced seasons, annual weather changes usually fixed the calendar; in warmer climates such as Southern Europe, Africa, and the Middle East, the moon was used to mark time.

Unfortunately, the cycles of the sun and moon do not synchronize well. A lunar year (consisting of 12 lunar cycles, or lunations, each 29½ days long) is only 354 days, 8 hours long; a solar year lasts about 365¼ days. After three years, a strict lunar calendar would have diverged from the solar calendar by 33 days, or more than one lunation.

The Muslim calendar is hence the only purely lunar calendar in widespread use today. Its months have no permanent connection to the seasons— Muslim religious celebrations, such as Ramadan, may thus occur at any date of the Gregorian calendar.

The phases of the moon have nonetheless remained a popular way to divide the solar year, if only because a 365¼-day year doesn't exactly lend itself to equal subdivision (the 71¼-day month has yet to find favor among menologists).

To compensate for the difference in the solar and lunar year, calendar makers introduced the practice of intercalation—the addition of extra days or months to the calendar to make it more accurate. The semilunar Hebrew calendar, consisting of twelve 29- and 30-day months, adds an intercalary month seven times every 19 years (which explains the sometimes confusing drift of Passover—and consequently Easter— through April and March).

Best of All Possible Calendars?

Despite its widespread use, the Gregorian calendar has a number of weaknesses. It cannot be divided into equal halves or quarters; the number of days per month is haphazard; and months or even years may begin on any day of the week. Holidays pegged to specific dates may also fall on any day of the week, and vanishingly few Americans can predict when Thanksgiving will occur next year.

Since Gregory XIII, many other proposals for calendar reform have been made. In the 1840s, philosopher Auguste Comte suggested that the 365th day of each year be a holiday not assigned a day of the week. The generic "Year Day" would allow January 1 to fall on a Sunday every year. Needless to say, this clever solution was not widely embraced.

The French Revolution also saw an attempt at the introduction of a new calendar. On October 5, 1793, the revolutionary convention decreed that the year (starting on September 22, 1792—the autumnal equinox, and the day after the proclamation of the new republic) would be divided into 12 months of 30 days, named after corresponding seasonal phenomena (e.g. seed, blossom, harvest).

The remaining five days of the year, called sans-culottides, were feast days. In leap years, the extra day, Revolution Day, was to be added to the end of the year. The Revolutionary calendar had no week; each month was divided into three decades, with every tenth day to be a day of rest. This straightforward calendar, however, perished with the Republic.