What this weekday calculator helps you do
The Weekday Calculator answers a simple date question with the extra calendar context that usually matters: the weekday name, weekday number, ISO weekday number, day of year, ISO week number, weekend status, and leap-year status.
Use it when a calendar view is inconvenient, when you need to check a date far away from today, or when you are validating date logic for schedules, reports, forms, or code.
How to read the result
A single date can be represented in several calendar systems. The calculator shows the common values side by side so you can copy the one your workflow needs.
| Result field | Meaning | Typical use |
|---|---|---|
| Weekday | The day name for the selected calendar date. | Events, birthdays, deadlines, historical checks. |
| Weekday number 0-6 | Sunday is 0, Monday is 1, and Saturday is 6. | JavaScript-style date checks and US-oriented systems. |
| ISO weekday 1-7 | Monday is 1 and Sunday is 7. | International schedules and ISO 8601 calendars. |
| Day of year | The date's position from 1 to 365, or 366 in a leap year. | Logs, reports, research notes, and data exports. |
| ISO week number | The ISO 8601 week containing the selected date. | Project planning, payroll periods, manufacturing weeks. |
| Leap year | Whether the selected date's year has February 29. | Checking annual schedules and date calculations. |
Weekday numbering systems
Weekday numbers are not universal. The calculator shows both common systems because a correct result in one system can look off by one in another.
0-6 format
Sunday starts the sequence at 0. This is the format many developers recognize from JavaScript date handling.
- 0 = Sunday
- 1 = Monday
- 2 = Tuesday
- 3 = Wednesday
- 4 = Thursday
- 5 = Friday
- 6 = Saturday
ISO 8601 format
Monday starts the sequence at 1 and Sunday ends it at 7. This is common in international business calendars.
- 1 = Monday
- 2 = Tuesday
- 3 = Wednesday
- 4 = Thursday
- 5 = Friday
- 6 = Saturday
- 7 = Sunday
Gregorian calendar assumptions
The calculator applies Gregorian calendar rules to the selected date. For modern dates, that is the calendar most users expect. For older dates, especially before October 15, 1582, historical records may have used the Julian calendar or a country-specific transition date.
Leap year rule used by the tool
- Years divisible by 4 are leap years.
- Years divisible by 100 are not leap years.
- Years divisible by 400 are leap years after all.
That is why 2024 and 2000 are leap years, while 1900 is not. Leap years affect every date after February 29 in that year.
When to use it
- Event planning: check whether a deadline, appointment, anniversary, or launch date lands on a weekday or weekend.
- Project reporting: convert calendar dates into ISO week numbers for weekly status, payroll, or manufacturing schedules.
- Historical research: get a Gregorian weekday while keeping the historical-calendar caveat visible.
- Programming checks: compare date-library output against an independent weekday, ISO week, and leap-year result.
- Operations planning: confirm day-of-year values used in logs, seasonal planning, or recurring data exports.
What it does not do
The tool reads the selected calendar date. It does not convert a timestamp between time zones, calculate country-specific public holidays, or count business days between two dates.
- Use a time-zone tool when the same instant could be a different date in another city.
- Use a business-day or work-hours tool when holidays, weekends, or work schedules need custom rules.
- Use historical sources when the exact local calendar system matters for older records.
Frequently Asked Questions
What is the difference between weekday numbers 0-6 and 1-7?
The 0-6 format is the common JavaScript and US-style numbering where Sunday is 0. The ISO 8601 format uses 1-7, where Monday is 1 and Sunday is 7.
Can I trust results for historical dates?
The calculator applies Gregorian calendar rules consistently. That is accurate for modern dates, but dates before October 15, 1582 may differ from records kept under the Julian calendar or from countries that adopted the Gregorian calendar later.
What are day of year and ISO week number useful for?
Day of year helps with scheduling, reporting, and scientific references. ISO week numbers are useful for project planning, manufacturing, payroll, and international business calendars.
Does this calculate holidays or business days?
No. This tool identifies weekday, weekend status, day of year, ISO week number, and leap year status for a selected date. It does not apply country-specific holidays or business-day calendars.
Can I copy or share a result?
Yes. Use Copy to save the current result summary, or Share to copy the exact calculator URL so someone else can open the same date and format.
Does leap year affect the weekday calculation?
Yes. Leap years add February 29, which shifts weekday positions for dates after that point. The calculator handles this automatically and also shows whether the selected year is a leap year.