Given that we are in a leap year this year (2020), it would be nice to know how to programmatically calculate this.

Luckily, this is a repeatable pattern that we can write some code for.

## So what is a leap year?

A `leap year` is a year that has 29 days in the month of February.

Astronomical years have a quarter of a day more than our calendar years that we follow, so to make sure this matches up continually, our calendar introduces an additional day every 4 years.

## Building a pattern

The pattern we can follow to make sure that we write our code correctly is:

```If the year is not divisible by 4 then it is a common year
Else if the year is not divisible by 100 then it is a leap year
Else if the year is not divisible by 400 then it is a common year
Else it is a leap year```

## Writing some code

We can easily turn this into code:

 ``````1 2 3 4 5 6 7 8 9 `````` ``````def isLeapYear(year): if year % 4 != 0: return False, "Common year" elif year % 100 != 0: return True, "Leap year" elif year % 400 != 0: return False, "Common year" else: return True, "Leap year" ``````

Our function above takes in a `year` variable and returns two variables. A `boolean` is it is a leap year or not and a `string` telling us what it is.

## Testing our code

Letâ€™s test it!

 `````` 1 2 3 4 5 6 7 8 9 10 11 `````` ``````print( isLeapYear(2020) ) # (True, 'Leap year') print( isLeapYear(1999) ) # (False, 'Common year') print( isLeapYear(1685) ) # (False, 'Common year') ``````