The calendar you are using this year can be reused. It will take a long time though, and there are exceptions. Let’s see which ones.

Maybe you never thought about it, but the **calendar** you are using this year **can be reused** several times. Not too frequently though, and there are exceptions.

In general, **the time that must pass** for a calendar to become valid again (therefore with the days of the week that coincide) is **28 years. **In 2020, for example, we could stick to the wall or put a 1992 calendar on the desk.

*A 2020 calendar can be reused in 2020: the days of the week of that year 28 years ago coincide with the days of 2020.*

If you are a collector of calendars, you can then take a ’92 calendar out of the drawer and use it as if it were from 2020 in total tranquility: from January 1st to December 31st, you will have no surprises! The days of the month will exactly match those of the week.

## 1992, 1964, 1936 … calendars that you can reuse in 2020!

And if you are a collector of ancient calendars, this year you can **also** safely use **a calendar from 1964, or from 1936! **They can be found in antique markets or on the Internet; give it a try, you will see that the days coincide with those of 2020.

## Why every 28 years?

At first you might think that a calendar should go back to being valid every 7 years. The basic problem, in fact, from year to year, is that the days of the week “slip”. For example, if the year previously January 1st was a Wednesday, this year will be a Thursday, and next year the same day will fall on a Friday.

We know this well, because **every year national holidays fall on different days of the week. **The years in which the holidays fall on Saturday, or worse on Sunday, are unfortunate! However, there are years in which they fall in such a way as to allow Bridges.

So in theory, every 7 years, things should go back “right.”

## The problem of leap years

The reason why a calendar repeats itself every 28 years and not every 7 years is that leap years are involved. **For example, 2020 is a leap year (with 366 days instead of 365: remember that in February we had 29 days and not 28)** . Leap years repeat every 4 years, and have been “invented” to correct an error that otherwise would accumulate over time ( we talked about it here ). So every four years there is a jump between the day of the week of the previous year and the current one.

Read also. Leap years: what are they and why do they exist?

For example, May 1, 2019 was a Wednesday, while in 2020 it falls on a Friday: in fact there was a leap year, **with February 29** (which falls every 4 years), which “messed up” a little ‘ the things. So here to be reusable, a calendar, **7 years are not enough but you have to wait up to 28 years. **Every 28 years, in fact, the days of the week realign. Just a few weeks ago we talked about how February 29 fell on Saturday in this 2020, and that this has **not happened … since 1992!**

## There is an exception to the 28-year rule

There is an exception to the 28-year rule: in the Gregorian calendar in fact, leap years are repeated every 4 years but **only the beginning of the century years divisible by 400 are leap years** . So, for example, 1900 was not a leap year while 2000 was (because it can be divided by 400). In 2100, therefore, the 28-year rule will break because that year will not be a leap year (you cannot divide 2100 by 400). At the moment, however, this is not our problem, since we are still in 2020.

## What calendar can we use in 2021?

Returning to us, **in 2021 we will be able to use a calendar 28 years earlier, then from 1993.** And in 2022? That of 1994. And so on. We can reuse the 2020 calendar in 2048. So if your calendar is beautiful and full of photos you don’t want to throw, put it in a drawer! It will come in handy … in 28 years.