Calendars help us keep the time of the day in sync with the time of nature. But Earth’s path around the Sun isn’t a neat whole number of days. Instead, it drifts unless we make minor changes. What are leap years? These modifications are their main component.

In these years, there is an extra day, February 29, to keep the times in sync with the seasons. What is a leap year, exactly? It’s a simple way to align the calendar with the stars. It ensures that the solstices and equinoxes happen at about the exact times every year and that history records, harvests, and holidays stay the same for hundreds of years.

The Leap Year Rule Explained

It’s easy to remember the new rule once you know why it’s there. From one March equinox to the next, there are 365.2422 days in a tropical year. If every year had 365 days, the calendar would be off by about one full day every four years, or about a quarter of a day every year.

To fix this, we add a leap day to most fourth years, but not all. In the Gregorian calendar, this is the basis for what are leap years.

In simple terms, the rule is:

• Rule 1: If you can divide the year by 4, it’s a leap year.
• Rule 2: If the year is less than or equal to 100, it is not a leap year.
• Rule 3: Except again, if the year is less than or equal to 400, it is a leap year.

Why is this exception two steps? Because adding a day every four years makes things a little too right. So, the calendar stays balanced over time, and the century and 400-year rules reduce the extra time. This explains why leap years exist in addition to “every fourth year.”

It’s easy to spot leap years with the help of the table below:

Year	Divisible by 4?	Divisible by 100?	Divisible by 400?	Leap year?	Reason
1996	Yes	No	—	Yes	Rule 1 applies
1900	Yes	Yes	No	No	Rule 2 cancels leap
2000	Yes	Yes	Yes	Yes	Rule 3 restores leap
2023	No	—	—	No	Not ÷ 4
2024	Yes	No	—	Yes	Rule 1 applies
2032	Yes	No	—	Yes	Every 4 years
2036	Yes	No	—	Yes	Every 4 years
2040	Yes	No	—	Yes	Every 4 years
2100	Yes	Yes	No	No	Rule 2 cancels leap
2400	Yes	Yes	Yes	Yes	Century year ÷ 400

This rule ensures that the average length of the calendar year stays very close to the length of the astronomical year, with only a few seconds of shift per year.

History of Leap Years

Julius Caesar created the Julian calendar in 45 BCE, marking the beginning of the history of leap years. It always added a leap day every fourth year, making the year about 365.25 days long. Even though this was a massive gain over the Roman systems that came before it, it was still 11 minutes behind the tropical year every year. Over hundreds of years, that small amount built up. The times of the spring equinox had moved back about ten days on the calendar by the 1500s.

In 1582, Pope Gregory XIII implemented the Gregorian change to stop the growing error. Two significant changes stopped the drift:

• Realignment of the calendar: Losing ten days moved the equinox back to where it should be in March.
• Changes to the leap-year rule: The years 1700, 1800, 1900, etc. would not be leap years unless divisible by 400, like 1600 and 2000.

These changes reduced the mistake to less than a day over 3,200 years. It was close enough for research, everyday life, and holiday celebrations. This change is what gave rise to the current understanding of what leap years are.

When Is the Next Leap Year

Based on today, 2025, the next leap year will be 2028. Then, remember the 4/100/400 rule; there will be leap years in 2032, 2036, and 2040. As you might expect, many places are planning special events for February 29. Some schools hold science shows on “leap day”, and cultural organizations hold events every four years.

Significant events that happen around the world can also happen during leap years. In this case, the Summer Olympics will happen in 2028. When is the next leap year? Knowing this helps you make extremely accurate plans for upcoming events, milestones, or multi-year plans.

Fun Facts About Leap Years

Here are some fun facts about leap years that demonstrate how this extra day has become ingrained in culture and the law:

• Leap day birthdays (leaplings): People born on February 29 usually celebrate on February 28 or March 1 if the year is not a leap year. This depends on local tradition or the law.
• Folklore and traditions: Some parts of Europe had traditions that switched roles on Leap Day. For example, offers started by women can look very different depending on where they are located.
• Contracts and paychecks: With February 29, annualized rates, day-count practices, and interest estimates in legal and financial deals can change.
• Cycles of the calendar: Years in the 2100s are not leap years unless divisible by 400. This means kids born in 2096 might not see another leap day until 2104.
• Winner is astronomy: The Gregorian rule is a reasonable middle ground. It is accurate enough and easy to use to keep seasonal markers (equinoxes and solstices) close to the exact dates on the calendar for thousands of years.

How to Check if a Year Is a Leap Year

Use this straightforward process to determine what leap years are if you need a quick diagnosis for coding, planning, or teaching:

Step-by-step method:

1. If the year is not divisible by 4 → Common year (not a leap year).
2. If the year is divisible by 4 but not by 100 → Leap year.
3. If the year is divisible by 100 but not by 400 → Common year.
4. If the year is divisible by 400 → Leap year.

Worked example:

1. Is 2100 a leap year? It’s divisible by 4 (yes) and 100 (yes), but not by 400 → not a leap year.
2. Is 2000 a leap year? Divisible by 4 (yes), 100 (yes), and 400 (yes) → leap year.

Python code snippet:

The short Python code checks the year you type in to see if it’s a leap year. The 4/100/400 rule tells it to put “Leap” for leap years and “Common” for other years.

def is_leap(year: int) -> bool:

    return (year % 4 == 0) and (year % 100 != 0 or year % 400 == 0)

# Examples:

for y in [1996, 1900, 2000, 2024, 2028, 2100]:

    print(y, “->”, “Leap” if is_leap(y) else “Common”)

Conclusion and Key Takeaways

The civil calendar stays very close to the Earth’s path thanks to changes called leap years. Without them, the seasons would change throughout the months, making farming, science, faith, and everyday life more difficult. The Gregorian rule strikes a beautiful mix between ease of use and accuracy.

Remember this quick way to find out: “Divisible by 4? Most likely leap. Divisible by 100? Most likely not. Divisible by 400? Without a doubt, leap. You can quickly find out future leap days for plans, projects, and questions by using this rule to explain what are leap years to anyone.

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