Leap Years and Week Numbers

One extra day, two steps forward

A common year has 365 days, which is 52 weeks and 1 day, so the same date moves one weekday forward from one year to the next. A leap year has 366 days, which is 52 weeks and 2 days, so dates after 29 February move two weekdays forward. 22 October 2026 is a Thursday, 22 October 2027 is a Friday, and 22 October 2028, a leap year, is a Sunday, two days after Friday.

This drift is the engine behind the week numbers. Because the weekday of 1 January changes every year, the ISO week structure changes as well: the Monday of week 1 sits on a different date each year, and in some years week 53 appears.

The leap year rule

A year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. So 2024 and 2028 are leap years, 1900 and 2100 are not, and 2000 and 2400 are. The rule gives an average year of 365.2425 days, which is very close to the length of the solar year.

The calculator uses this rule for every year from 1 to 9999. The last row of the third tab tells you whether the year is a leap year, next to the number of days and the number of ISO weeks.

Leap years and 53 weeks

A leap year has 53 ISO weeks if it begins on a Wednesday or a Thursday. 2020 began on a Wednesday and has 53 weeks, and 2032 begins on a Thursday and will have 53. A leap year that begins on any other day has 52 weeks. 2024 began on a Monday and 2028 begins on a Saturday, so both have 52.

Of the 71 long years in each 400-year cycle, 27 are leap years, and 44 are common years that begin on a Thursday. A common year has 53 weeks only when it begins on a Thursday, while a leap year has two chances, Wednesday and Thursday.

The 400-year cycle

The Gregorian calendar repeats exactly every 400 years. One cycle has 146,097 days, which is exactly 20,871 weeks. That is why the weekday and week number of a date in 2026 are the same as for the same date in 2426. The pattern of leap years, long years and week 1 starts all repeat together.

This fact makes the week count predictable far into the future. A program does not have to store a table. It can calculate the answer for any year from a rule, and a rule that has been checked once is correct for all years in range. The calculator on this site was tested against an independent implementation across the whole supported range.

Days, weeks and 29 February

The leap day sits inside a week like any other day. 29 February 2028 is a Tuesday in ISO week 9, and it has the same week number as the other days of that week, from Monday 28 February to Sunday 5 March 2028. No special case is needed.

The leap day does change counts. The number of days between 1 March in two consecutive years is 365 or 366, and so is the number of days in the ISO weeks of those years. To count the days between two dates, use the first tab of the date calculator on this site, which includes the leap days in the range.

Week numbers across a leap day

Because the leap day only adds a day, the week numbers after it do not need adjustment. Week 10 of 2028 starts on Monday 6 March, and week 10 of 2027 starts on Monday 8 March. The difference of two days between consecutive years is typical for a leap year: the Monday of any week moves one date earlier in a common year and two dates earlier in a leap year.

The Monday of week 1 moves too. In 2026 it is 29 December 2025, in 2027 it is 4 January 2027, in 2028 it is 3 January 2028, and in 2029 it is 1 January 2029. A leap day in 2028 explains why the 2029 start moves two days back instead of one.

Checking a leap-year date

Here is a short test of your understanding. In 2028, 1 January is a Saturday, and the year has 366 days. Which weekday is 31 December 2028? 366 days is 52 weeks and 2 days, so it is two days after Saturday, a Sunday. The ISO week of Sunday 31 December 2028 is week 52, ending the year, and the next week, week 1 of 2029, begins on Monday 1 January 2029.

You can do the same for any leap year, and then compare your answer with the calculator. If both agree, you have understood how the leap day travels through the weekdays and the week numbers.

A quick way to remember

Remember the pattern: after a common year, the weekday of a date moves forward one day; after a leap year, two. A year is long when it has 53 Thursdays. Combine the two and you can work out the week structure of any year from the weekday of 1 January and whether the year is a leap year.

For checks, trust the arithmetic more than a calendar printed from memory. Use the calculator to confirm a date, a week or a year, and use the rule to understand why the answer is what it is.