Day of Week Calculator
A day of week calculator turns any calendar date into its correct weekday name and ISO weekday number in one step. Enter a date such as a birthday, a contract deadline, or a historical event, and the tool returns the weekday name (Sunday through Saturday) along with the ISO 8601 weekday number, where Monday is 1 and Sunday is 7. It works from the calendar date alone, using proleptic Gregorian calendar rules, so it treats every entry as a calendar day rather than a moment tied to a specific time zone or clock time. People use it to double-check invitation dates, confirm which weekday a recurring bill or lease payment lands on, verify historical trivia, or plan around a specific weekday months or years in advance.
Quick answer
Enter a date in YYYY-MM-DD format, or pick one from the date field.
What this tells you
- •Enter a date in YYYY-MM-DD format, or pick one from the date field.
- •The result shows the weekday name and the ISO weekday number, where Monday equals 1 and Sunday equals 7.
- •Calculations use proleptic Gregorian calendar logic, so the same rules apply to dates before and after the calendar's 1582 introduction.
- •The tool reads the date only. It does not factor in a time of day, a time zone, or daylight-saving changes.
- •Leap years are handled automatically, so February 29 in a leap year returns a normal weekday result.
How to Use
- 1Select a date using the date field, or type it directly in YYYY-MM-DD format.
- 2Click Calculate Day to run the lookup.
- 3Read the main result for the weekday name, such as Tuesday or Saturday.
- 4Check the ISO Weekday value if you need the numeric form used by scheduling systems, spreadsheets, or ISO 8601 date standards.
- 5Repeat with a new date any time you need to check another day, such as comparing several possible meeting dates.
How It Works
Formula
Weekday index = (total days from a fixed Gregorian reference point) mod 7, mapped to a weekday name and ISO weekday numberThe calculator converts the entered year, month, and day into a single Gregorian calendar day count using proleptic Gregorian rules, the same extended rules that ISO 8601 and most software calendars rely on. Dividing that day count by 7 and reading the remainder places the date at a fixed position in the seven-day week cycle. That remainder maps to a weekday name, from Sunday through Saturday, and then converts to an ISO weekday number by shifting Sunday from position 0 to position 7 while Monday through Saturday keep their 1 through 6 values. Because the math only needs the calendar date, no clock time or time zone offset enters the calculation.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Find the weekday for a birthday
May 9, 2026 falls on a Saturday, which the tool reports as ISO weekday 6. This kind of check helps confirm whether a birthday, anniversary, or reminder lands on a weekend before invitations or plans go out.
Check a leap day
February 29, 2024 is a leap day, and it falls on a Thursday, ISO weekday 4. Leap days only exist in leap years, so this example confirms the tool correctly accepts February 29 as valid and calculates the weekday the same way it would for any other date.
Confirm a holiday's weekday
Christmas Day 2026 lands on a Friday, ISO weekday 5, which puts it next to the weekend for a longer break. Retail schedules, delivery cutoffs, and office closures often depend on knowing this well in advance.
Verify a historical date
The Apollo 11 moon landing on July 20, 1969 occurred on a Sunday, which the tool reports as ISO weekday 7. This kind of lookup is common for trivia questions, history assignments, or fact-checking a date against event records.
Check the first day of the millennium
January 1, 2000 fell on a Saturday, ISO weekday 6. Millennium and century transitions are frequent lookups because they combine a leap year, a new decade, and a round number that people want confirmed.
Plan around a far-future date
June 15, 2050 falls on a Wednesday, ISO weekday 3, decades before the date arrives. Proleptic Gregorian calendar rules keep producing consistent results for dates far in the future, which helps with long-range planning such as insurance terms, loan maturities, or multi-decade contracts.
ISO Weekday Number Reference
How each weekday name maps to its ISO 8601 weekday number and the common US calendar position.
| Weekday name | ISO weekday number | Common US position (Sun=1) |
|---|---|---|
| Monday | 1 | 2 |
| Tuesday | 2 | 3 |
| Wednesday | 3 | 4 |
| Thursday | 4 | 5 |
| Friday | 5 | 6 |
| Saturday | 6 | 7 |
| Sunday | 7 | 1 |
ISO 8601 always starts the week on Monday, while many US calendars and spreadsheet functions start the week on Sunday.
Common mistakes
- Entering an invalid calendar date, such as April 31 or February 30, which cannot resolve into a real day.
- Expecting a time zone or time-of-day answer, when the tool works from the calendar date alone.
- Assuming the tool switches to the Julian calendar for dates before 1582. It always applies proleptic Gregorian rules instead.
- Mixing up ISO weekday numbers with the common US convention, where Sunday is often treated as day 1 instead of day 7.
- Forgetting that leap years shift which weekday later dates in the same year fall on, compared with a non-leap year.
- Typing the date in the wrong field order, such as DD-MM-YYYY, when the tool expects YYYY-MM-DD.
Embed this calculator on your site
Drop this single line where you want the calculator to appear. It is responsive, mobile-friendly, resizes automatically, and is free to use with attribution.
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