Ah, the summer’s over and now it’s September! How do we know? Easy – look at the calendar.

It wasn’t always that simple, though. Back in September 1752, all along the colonies of the East Coast, people went to bed on Wednesday the 2nd and woke up the next morning on Thursday the 14th.

What happened??

Great Britain switched to the Gregorian calendar, that’s what. But how did they lose almost two weeks? Well, it all started with the old calendar …

A calendar year measures the time it takes for the sun to appear in the same position in the sky when viewed through a telescope. During this time, Earth completes one orbit around the sun, and we experience a full cycle of seasons.

However, because Earth rotates in a peculiar way, like a wobbling top, it doesn’t take a uniform amount of time from one year to the next for us to see the sun in the same position against the backdrop of stars. This irregularity makes it difficult to synchronize our calendar days with the seasons over a span of centuries.

To realign their calendar with the seasonal cycle, the Romans used to insert extra months into their year. Thus, 46 BC was a long, long year: 445 days! However, it became known as “the last year of confusion” because, after returning from his Nile cruise with Cleopatra, Julius Caesar put his foot down. He called in some experts and authorized a calendar reform.

The Julian year should sound familiar to us: 12 months, 365 days, and a leap day added to February every four years. To do away with the ad hoc system of inserting corrective days into every year, Caesar’s calendar borrowed from Roman and Egyptian calendars as well as Greek astronomy. It lasted for centuries, all the way through to the period of European settlement in the Americas.

Even so, thanks to its regularized leap years, the Julian calendar actually gained three days every 400 years. In medieval and Renaissance Europe, this meant that Easter kept drifting further and further away from the spring equinox. The leaps became a problem.

To correct for this gradual expansion of the year, a new system had to get rid of those three accumulated days. Enter the Gregorian calendar.

In 1582, Pope Gregory XIII introduced the calendar we use today, in which a leap year must be exactly divisible by 4 as well as 400. Thus, the years at the end of some centuries didn’t leap. The years 1600 and 2000 did; but 1700, 1800 and 1900 didn’t; and 2100 won’t. By controlling the leaps this way, the calendar gains only one day every three millennia, approximately. That’s as precise as we can get for now.

By the time the calendar was introduced, Easter was 10 days out of alignment with the prescribed traditional date. Gregory’s solution? Just hit delete. Thus, in the countries that switched, October the 4th was followed by the 15th, and then life went on.

Most of Catholic Europe adopted the new calendar, but Protestant states – including Denmark, Sweden, and Britain – held out for hundreds of years. The effect is interesting when we consider authors William Shakespeare and Miguel Cervantes. Both died on April 23, 1616, but Britain was still on the old calendar whereas Spain had reformed. So Cervantes actually died 10 days before Shakespeare.

Both calendars were in simultaneous use for almost two centuries, designated “Old Style” and “New Style” respectively. It wasn’t as confusing as in Roman times, but dual dating was indeed the order of the day. In fact, Russia switched only in 1918; and Greece, the very last holdout, waited until 1923.

Today, although many holy days and festivals are still calculated using ancient systems, the Gregorian calendar predominates. It is the world’s most widely used non-religious calendar.

So, now, what day is it again?

Well, someone did propose yet another calendar reform back in the 19th century, but until that happens, today is probably the date you thought it was.