Quick answer
The weekday of any date is found from the number of days since a known date. 29 February 2024 was a Thursday and the 60th day of the year.
Enter a date. The calculator returns the day of the week and the day of the year.
Related tools: Day of the Year Calculator: Which day of the year a date falls on. Week Number Calculator: ISO 8601 week number of a date. Leap Year Calculator: Check whether a year is a leap year.
How to use the Day of the Week Calculator
- Pick date.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Date | A calendar date | — |
Formula and method
Formula
weekday = (days since 1970-01-01 + 4) mod 7 (Sunday = 0)
Exact integer day arithmetic in UTC; no daylight-saving or time-zone effects.
1 January 1970 was a Thursday; weekday follows from the day count (proleptic Gregorian).
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Date = 2024-02-29.
| Result | Value |
|---|---|
| Day of the week | Thursday |
| Day of the year | 60 |
Example 2
Inputs: Date = 2000-01-01.
| Result | Value |
|---|---|
| Day of the week | Saturday |
| Day of the year | 1 |
Example 3
Inputs: Date = 1969-07-20.
| Result | Value |
|---|---|
| Day of the week | Sunday |
| Day of the year | 201 |
The first line is the headline result (day of the week). Below it the day of the Week Calculator lists day of the year, which describe the same case from other angles. Exact integer day arithmetic in UTC; no daylight-saving or time-zone effects.
How it works
Counting days from 1 January 1970 (a Thursday) and taking the remainder after dividing by 7 gives the weekday. 29 February 2024 is the 60th day: 31 days in January plus 29 in February. Leap years are handled by the Gregorian rule, divisible by 4 except century years not divisible by 400.
Old dates
Dates before 1582 use the proleptic Gregorian calendar, extending today's rules backwards. Many countries used the Julian calendar then, so historical weekdays may differ from what contemporaries recorded.
Day of the Week from different start dates
The same calculator run from 1 January of four consecutive years.
| Date | Day of the week | Day of the year |
|---|---|---|
| 2026-01-01 | Thursday | 1 |
| 2027-01-01 | Friday | 1 |
| 2028-01-01 | Saturday | 1 |
| 2029-01-01 | Monday | 1 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Date: entering "2024-02-30" shows "Date must be a valid date." beside that field and no result.
Choosing between the Day of the Week Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Day of the Year Calculator](/date-calendar/day-of-the-year-calculator/) | date | Day of the week |
| [Week Number Calculator](/date-calendar/week-number-calculator/) | date | ISO week number |
| [Leap Year Calculator](/date-calendar/leap-year-calculator/) | year | Result |
| [Days Until Labor Day](/date-calendar/days-until-labor-day/) | Days Until Labor Day | |
| [Days Until Father's Day](/date-calendar/days-until-fathers-day/) | Days Until Father's Day |
Background: dates and times
Date and time results depend on the conventions a tool uses: whether the end date counts, whether weekends and holidays are skipped, how months of different lengths are handled and whether time zones are applied. This page states its convention, and the worked examples show it.
Dates use the Gregorian calendar and calendar days unless the page says otherwise; check deadlines against the rule in your contract or regulation.
Assumptions and limitations
- Dates before 1582 use the proleptic Gregorian calendar
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for date to confirm this.
Frequently asked questions
Which day was I born?
Enter your birth date.
Is 2100 a leap year?
No, century years need divisibility by 400.
How many days are in a leap year?
A leap year has 366 days, with 29 days in February.
What does the Day of the Week Calculator calculate?
Find the weekday of any date. It returns Day of the week and Day of the year.
What formula does the Day of the Week Calculator use?
weekday = (days since 1970-01-01 + 4) mod 7 (Sunday = 0). 1 January 1970 was a Thursday; weekday follows from the day count (proleptic Gregorian).
What do I need to enter in the day of the Week Calculator?
You enter Date.
How is the result rounded or limited?
Exact integer day arithmetic in UTC; no daylight-saving or time-zone effects. Dates before 1582 use the proleptic Gregorian calendar.
What is an example of a day of the Week result?
With date 2024-02-29, the day of the week is Thursday.
Related calculators
- Day of the Year Calculator — Which day of the year a date falls on.
- Week Number Calculator — ISO 8601 week number of a date.
- Leap Year Calculator — Check whether a year is a leap year.
- Days Until Labor Day — Days until US Labor Day (first Monday of September).
- Days Until Father's Day — Days until US Father's Day (third Sunday of June).
- Days Until Mother's Day — Days until US Mother's Day (second Sunday of May).
Sources
- ISO 8601-1: Date and time, representations for information interchange (calendar dates, ordinal dates and week dates)
- Gregorian calendar reform, papal bull Inter gravissimas (1582): the leap-year rule used for all dates
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.