Today is February 29. Unlike every other day in the year, this day only shows up in the calendar once every four years… but why? To understand this, we’re going to use a little bit of history and astronomy to get to the calendar that most of the world uses today.
First, let’s talk about calendars. While most of us use what is called the Gregorian calendar (which includes 365 days, 12 months, 52 weeks, etc.), there are actually hundreds of different types of calendars that have been used throughout history. Each of these calendars generally falls into one of the following four categories: lunar , solar , lunisolar , or seasonal  (see our Footnotes for differences between these types.)
Formal calendars date back to the Neolithic period, but timekeeping itself likely predates written records. Most early calendars were dictated by naturally observable/regular phenomena, such as solar and lunar patterns. The calendar we use today found its roots in the Roman Empire, although the earliest incarnation, based on lunar cycles, had only 10 months and started in March.
The Julian calendar, an adapted Roman calendar, followed in the 1st century AD and has 12 months and 355 days. The Julian calendar was named after Julius Caesar and was created with the help of Greek mathematicians and astronomers. It is still used today by the Eastern Orthodox Church and the Berbers.
The Julian calendar is very similar to the modern calendar with 12 months, and even has leap years where there is a 29th day of February! However, the math used to dictate leap years would actually make today March 1, 2020 if you followed this calendar. The Julian calendar fell out of favor over time, because — as time passed, Easter fell out of alignment with the March equinox due to an erroneous placement of leap years (every three years instead of every four). The calendar most of us use today is the fixed Gregorian calendar, which is named after Pope Gregory XIII, and was created as a correction to the Julian calendar.
So, why did Pope Gregory XIII make an adjustment to the Julian calendar? This is where astronomy comes in. Both the Julian and the Greogorian calendars are based off of how many days it takes the Earth to make one full rotation around the Sun, but there’s some tricky math involved. It takes the Earth 365.2425 days to get around the Sun which, as you can imagine, might cause some issues with rounding. For example, if calendars followed this decimal exactly, we would have one day each year that was exactly 5 hours, 48 minutes, and 20 seconds long!
In order to fix these issues, leap years in the Gregorian calendar occur when:
- The year must be evenly divided by 4;
- If the year can be evenly divided by 100, it is NOT a leap year, unless;
- The year is evenly divisible by 400
Together, these rules provide a more accurate way to divvy up time and still respect the natural world — at least in the view of most Westerners. Aren’t calendars fun? Happy leap day!
 Lunar calendars are based on the monthly cycles of the moon. Moon cycles last approximately 29.5 days, and a lunar year is approximately 354 days long. Traditional lunar calendars are still in use for many cultural and religious groups, such as the Islamic calendar, the Chinese calendar, and the Hindu calendar.
 Solar calendars are calendars whose dates correspond with the Sun’s position to the stars. The oldest solar calendars are the Julian and Coptic calendars. There are two types of solar calendars: tropical solar and sidereal solar.
 Lunisolar calendars, as the name suggests, rely on both lunar phases and solar dates. Lunisolar calendars are also developed differently, depending on whether the solar dates are from a tropical solar calendar or a sidereal solar calendar.
 Seasonal calendars rely on changes in the environment rather than astronomical queues like the lunar, solar, and lunisolar calendars.