2017 – this year lasts 365 days. In 2016, there were 366 days because it was a *leap year*. 2020, 2024, 2028… are also leap years but 2100 isn’t. Why?

A leap year must be divisible by 4 but not by 100 except if it is also by 400. Whaaaat?

A solar year consists of exactly 365 days, 5 hours, 48 minutes and 46 seconds. That’s why some years – about ** every four years*** – have one additional day, that is 366 in total. However, if one of every four years were a leap year, we would end up ahead of the solar year. Therefore, every 100 years (almost), there are 365 days and not 366 in a year.

However, it hasn’t always been the case.

We’re currently using the *Gregorian calendar*, which Pope Gregory XIII established in 1582 to correct the difference that had built up. Before that, the calendar had already been changed several times to rectify the gap that had appeared between seasons and calendar.

One of the most striking adjustments that took place was in 46 B.C., known as ** the year of confusion**.

Before that, the Roman calendar was based on the phases of the moon: there were 29 days in a month and 12 months in a year, which, after centuries, added up to a huge gap. Consequently, in 46 B.C., Julius Caesar decided to correct it by adding a bunch of days; that year lasted 456 days!

He implemented a system that was similar to ours except for secular years (1000, 1100, 1200,…): these were all leap years, which was modified by Gregory XIII in 1582.

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** Every 4 years*

Structure = every + # + noun:

- They travel to Canada
.*every 2 years* - You need to take your medicine
.*every 8 hours*

Note that for “every + 2 + noun”, you can also say “every other + noun”:

- They travel to Canada
.**every other year**