Leap Years and the Calendar: How the Rules Fit Together

Why leap years exist

The calendar tries to keep step with the seasons, and the seasons follow the Earth's orbit around the Sun. One full cycle of the seasons takes about 365.2422 days, not a whole number. If every calendar year had exactly 365 days, the calendar would slip by almost a quarter of a day every year, about one day every four years. After a century, the seasons would arrive more than three weeks "late" by the calendar.

Adding a day every fourth year fixes most of that drift. A calendar with a leap day every four years averages 365.25 days per year. That is slightly too long, by about 0.0078 days a year, which adds up to roughly one day every 128 years. The modern rule trims that remaining error too.

The 4 / 100 / 400 rule

In the Gregorian calendar, used for civil dates in most of the world, a year is a leap year if:

it is divisible by 4, except
years divisible by 100 are not leap years, except
years divisible by 400 are leap years after all.

Year	÷ 4?	÷ 100?	÷ 400?	Leap year?
2024	Yes	No	No	Yes
2026	No	No	No	No
1900	Yes	Yes	No	No
2000	Yes	Yes	Yes	Yes
2100	Yes	Yes	No	No
2400	Yes	Yes	Yes	Yes

In every 400 years there are 100 multiples of 4, minus the 3 century years that are not multiples of 400, so 97 leap years. The average year is therefore 365 + 97/400 = 365.2425 days, within about 0.0003 days of the seasonal year. That remaining gap amounts to about one day in roughly 3,000 years.

The year 2000 was a leap year, which surprised some older software that only knew the "divisible by 100" exception. The next century year, 2100, will not be one. Between 2001 and 2100 there are 24 leap years: 2004, 2008 and so on up to 2096.

The Gregorian calendar was introduced in 1582. To correct the drift that had built up under the older Julian calendar, ten dates were skipped: Thursday 4 October 1582 was followed by Friday 15 October 1582. Countries switched at different times over the following centuries, so historical dates sometimes need care.

A neat fact about the 400-year cycle

400 Gregorian years contain 400 × 365 + 97 = 146,097 days, and 146,097 ÷ 7 = 20,871 exactly. So the whole calendar, including weekdays, repeats every 400 years. 14 November 2426 will fall on the same weekday as 14 November 2026.

Born on 29 February

Since 97 of every 146,097 days are 29 February, the chance that a given day is a leap day is about 1 in 1,506. Someone born on 29 February 2000 has had an actual birthday on the calendar in 2004, 2008, 2012, 2016, 2020 and 2024: six times by 2026, even though they turned 26 that year.

In common years the date does not exist, so people and organisations choose to treat either 28 February or 1 March as the birthday. Which one is used for official purposes depends on local rules. The age guide explains how this affects counting age, and the age calculator handles leap-day births for you.

Days in each month

Seven months have 31 days, four have 30, and February has 28 or 29. Two classic memory aids help.

The rhyme: "Thirty days hath September, April, June and November", with all the rest having 31 except February.
The knuckle method: make a fist and count the months across your knuckles and the dips between them. January is the first knuckle, February the first dip, March the next knuckle, and so on. Knuckles are 31-day months; dips are 30-day months or February. When you reach July on the last knuckle, start again on the first knuckle for August, which is why July and August both have 31 days.

Month	Days	Day of year at month end (common year)
January	31	31
February	28 (29)	59
March	31	90
April	30	120
May	31	151
June	30	181
July	31	212
August	31	243
September	30	273
October	31	304
November	30	334
December	31	365

The last column gives the day number of any date quickly. 14 November 2026 is day 304 + 14 = 318 of the year. In a leap year, add 1 for any date after February.

How dates move through the week

A common year has 365 days, which is 52 weeks and 1 day. So a date moves forward one weekday each year. A leap year has 366 days, 52 weeks and 2 days, so a date moves forward two weekdays when 29 February falls in between.

14 November 2026 is a Saturday.
14 November 2027 is a Sunday (one step).
14 November 2028 is a Tuesday (two steps, because 29 February 2028 lies in between).

The same idea finds the weekday a given number of days ahead: divide the number of days by 7 and move forward by the remainder, as shown in the date guide.

Finding the weekday of a date in your head

A popular method relies on the fact that certain easy dates always share the same weekday within a year. These "anchor" dates are 4/4, 6/6, 8/8, 10/10 and 12/12 (4 April, 6 June, 8 August, 10 October, 12 December), plus 9 May, 5 September, 11 July, 7 November, 14 March and the last day of February.

In 2026 all of these fall on a Saturday. To find the weekday of any 2026 date, start from the nearest anchor and count the difference in days, keeping only the remainder after dividing by 7.

Example: 25 December 2026. The nearest anchor is 12 December, a Saturday. 25 − 12 = 13 days, and 13 = 7 + 6, so move forward 6 days: Saturday → Friday.

Example: 1 July 2026. The anchor 11 July is a Saturday. 1 July is 10 days earlier; 10 = 7 + 3, so go back 3 days: Saturday → Wednesday.

For another year, move the anchor weekday forward by one for each common year and by two for each leap year. 2027 anchors are on Sunday, and 2028 anchors are on Tuesday, matching the 14 November example above. In a leap year, use 29 February as the February anchor.

Common mistakes

Using only the "every 4 years" rule. It is wrong for 1900 and 2100.
Assuming a year is always 365 days. Spans that include 29 February are one day longer.
Forgetting the leap day in date counts. From 1 February to 1 March is 29 days in 2028 but 28 in 2026.
Trusting old spreadsheets for early dates. Some spreadsheet programs treat 1900 as a leap year for historical compatibility, so date counts across early 1900 can be off by one.
Mixing calendars in history. Dates before a country switched to the Gregorian calendar may be recorded in the older system.

FAQ

Is 2026 a leap year?
No. 2026 is not divisible by 4. The previous leap year was 2024 and the next is 2028.

How many days are in a leap year?
366. February has 29 days instead of 28; every other month stays the same.

Are leap seconds related to leap years?
No. Leap seconds are occasional one-second adjustments to clock time, made for a different reason. They do not change the number of days in a year.

Can I check my answers?
Yes. The date calculator counts days between any two dates and includes leap days automatically.

Open the calculator: Date Calculator →