Day of the Week Calculator: Weekday, ISO Week, Day of Year

How to use it

Type the date in the Date box, or pick it from the calendar.
Read Day of the week, then ISO week and Day of the year.
For a date before or after today, read Days from today.
Open The same date in the next ten years to see the weekday of the same date in each coming year.

How the weekday is worked out

The weekday comes from Zeller's congruence, a formula named after Christian Zeller. It adds the day of the month, the month and the year in one sum, and the remainder after dividing by 7 gives the weekday. January and February count as months 13 and 14 of the year before.

The formula gives 0 for Saturday in its own count. This page shifts the result so that 0 means Sunday, which is how the calendar is counted here. Writing 5J instead of −2J gives the same remainder, because 5 and −2 differ by 7.

h = (q + ⌊13 × (m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5J) mod 7

q  day of the month
m  month, with January and February as 13 and 14 of the year before
K  year within its century (year mod 100)
J  the century (year ÷ 100, rounded down)
h  0 = Saturday, 1 = Sunday ... 6 = Friday

Day of the year and ISO week

The day of the year counts from 1 January, which is day 1. The ISO week number follows ISO 8601: weeks run from Monday to Sunday, and week 1 is the week that holds the year's first Thursday. So 29 December 2025 is in week 1 of 2026, and 1 January 2027 is in week 53 of 2026.

What this calculator does not cover

Dates before the Gregorian calendar was adopted in each country. This page applies the Gregorian rules to every date, so its weekday for an older date can differ from the weekday people used then.
Time zones and the time of day: a date is a calendar day, so the weekday does not change with your location.

Worked example: 20 July 1969

Month 7 and year 1969 need no adjustment, so q = 20, m = 7, K = 69 and J = 19. The terms are: 20 (the day); ⌊13 × 8 ÷ 5⌋ = ⌊20.8⌋ = 20; 69; ⌊69 ÷ 4⌋ = 17; ⌊19 ÷ 4⌋ = 4; and 5 × 19 = 95.

Their sum is 20 + 20 + 69 + 17 + 4 + 95 = 225. Dividing 225 by 7 gives 32 with a remainder of 1, so h = 1, which is Sunday. The result is Sunday, 20 July 1969.

The same date is day 201 of the year and falls in ISO week 29 of 1969.

Output	Value
Day of the week	Sunday
Day of the year	201
ISO week	Week 29, 1969

Frequently asked questions

How do I find out what day of the week a date was or will be?

Type the date in the Date box. The Day of the week output names the weekday in your language. It is worked out with Zeller's congruence, which needs only the day, the month and the year.

What is an ISO week number?

It is the international standard for week numbers, ISO 8601. Weeks run from Monday to Sunday, and week 1 holds the first Thursday of the year. A year has 52 or 53 ISO weeks.

Why does 1 January sometimes belong to the last week of the year before?

When 1 January is a Friday, Saturday or Sunday, the Monday-to-Sunday week that holds it has its Thursday in the year before. So that week is the last week of the previous year.

Does the time zone of my device change the weekday?

No. The date is treated as a calendar day with no time and no time zone, so the weekday stays the same wherever you are and whatever the clocks say.

Why do some years in the next-years table show no date?

A 29 February date exists only in a leap year. In the other years the table shows No such date, so you can see which years do not have that day.

Does the page send the date I enter anywhere?

No. The weekday and the week are calculated in your browser, and the date you type is not stored or sent to us.

Sources

Figures and text last checked on Tuesday, November 10, 2026.

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