Birthday Day Calculator
Birthday day calculator works out Day of the week and ISO weekday number (Monday = 1) from Date, Count anniversaries from and How many anniversaries to list. The default figures on this page give day of the week of Sunday; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
4 steps
- Zeller's congruence for 1969-07-20: q=20, m=7, K=69, J=19: (20 + floor(13*(7+1)/5)=20 + 69 + floor(69/4)=17 + floor(19/4)=4 + 5*19=95) mod 7 = 1, and h = 1 counts from Saturday, so the day is Sunday.
- Day of the year: 201 of 365 (1969 is not a leap year), leaving 164 days.
- ISO week date: 1969-W29-7. The week belongs to 1969 because that is the year holding its Thursday.
- Anniversaries are listed from 2026-09-14 forward; a common year advances the weekday by one (365 = 52*7 + 1) and a leap year by two.
Coming anniversariestable
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the birthday day calculator calculates
The weekday any date falls on, with the hand working (Zeller’s congruence), the day of the year, the days left in the year, the quarter, the ISO week and the ISO week-numbering year, which is not always the calendar year. In birth-date mode it also lists the weekday of the coming anniversaries.
How to use this birthday day calculator
Enter Date, Count anniversaries from and How many anniversaries to list. Day of the week updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Day of the week, ISO weekday number (Monday = 1) and US weekday number (Sunday = 1).
The method follows ISO 8601-1:2019, Date and time - Representations for information interchange, Christian Zeller, "Kalender-Formeln", Acta Mathematica 9 (1887), 131-136 and Howard Hinnant, chrono-Compatible Low-Level Date Algorithms.
What you can compare
Change one input at a time to see how it affects Day of the week, ISO weekday number (Monday = 1) and US weekday number (Sunday = 1). The inputs you can vary are Date, Count anniversaries from and How many anniversaries to list.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to calculate birthday days?
The weekday any date falls on, with the hand working (Zeller’s congruence), the day of the year, the days left in the year, the quarter, the ISO week and the ISO week-numbering year. Open “How this is worked out” beside the result to see each step for your own figures.
What is the math trick to calculate birthday dates?
It needs Date, Count anniversaries from and How many anniversaries to list. Nothing else is required, and the figures stay in your browser.
How to find birthday date trick?
The result gives Day of the week, ISO weekday number (Monday = 1), US weekday number (Sunday = 1), Day of the year, Days left in the year and Days in that year, each labelled with its unit so the figure can be read on its own.
What information do I need for the birthday day calculator?
You need Date, Count anniversaries from and How many anniversaries to list. The page opens with default figures that give day of the week of Sunday, so you can see the scale before entering your own.
What does this birthday day calculator show?
It shows Day of the week, ISO weekday number (Monday = 1), US weekday number (Sunday = 1), Day of the year, Days left in the year, Days in that year, ISO week number and ISO week-numbering year, with the working and any warnings for your entries.
Can I check how the birthday day calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.