Find the day of the week for any valid Gregorian date, plus ISO day number, day of year, ISO week, birthday weekday lookups, weekend status.
Last updated
Weekday finder for birthdays, future dates, and historical checks Enter any valid Gregorian date to find the day of the week, ISO week number, day of year, quarter, weekend status, and the Monday-to-Sunday ISO week span.
Quick date checks
Day of week
Monday
Monday, 1 January 2024 is a weekday in ISO week 1 of 2024.
ISO day number
1
Monday is 1; Sunday is 7.
Day of year
1
365 days remain in the year.
ISO week
W1
ISO week-year 2024.
Month position
1st Monday
January has 31 days.
Quarter
Q1
Leap year.
Weekend status
Weekday
Based on Saturday and Sunday weekends.
ISO week span This date sits in the Monday-to-Sunday ISO week from 2024-01-01 through 2024-01-07. This is useful when a planner, payroll sheet, school term, or international record refers to week numbers instead of calendar months.Same weekday nearby The previous Monday was 2023-12-25, and the next Monday is 2024-01-08. Use those dates when you need the nearest matching weekday around a deadline or birthday.
Copyable date facts
These rows keep the weekday, ISO week number, day-of-year, and weekend status together for birthday lookups, historical notes, and schedule checks.
Day of week calculator guide: weekday finder, birthday lookup, ISO week
The day of week calculator tells you which weekday a date falls on, from Monday through Sunday. It works as a weekday finder, birthday weekday lookup, future-date checker, historical date checker, day-of-year calculator, ISO week number calculator, and weekend-status check for any valid Gregorian date.
What this day of week calculator returns
Most people arrive with a direct question such as what day was I born on, what day of the week was this date, or what day will a future deadline fall on. A useful answer should be more than a single weekday name, because the surrounding date facts often matter for planning, records, and verification.
This calculator therefore returns the weekday, ISO day number, day of year, days remaining in the year, ISO week number, ISO week-year, Monday-to-Sunday ISO week span, quarter, leap-year status, weekend status, and the previous and next matching weekday. Those extra facts make it easier to copy the result into a planner, research note, payroll sheet, school calendar, or birthday record.
How the weekday is calculated
The calculation uses strict Gregorian calendar arithmetic on the exact ISO-style date you enter. The date is parsed as a calendar date, not as a local-time timestamp, so the weekday does not shift when a browser, server, or user is in a different time zone.
Behind the scenes, weekday algorithms such as Zeller's congruence, Sakamoto's algorithm, and Doomsday-style methods all rely on the same idea: calendar dates repeat in a predictable 400-year Gregorian cycle. This page uses the JavaScript calendar engine with strict validation and UTC date construction so impossible dates such as 2024-02-31 are rejected rather than silently normalized.
ISO weekday = Monday 1 through Sunday 7
The calculator also shows the ISO day number because many international planning systems use Monday as the first day of the week.
Day of year = days before this month + day of month
February contributes 29 days in leap years and 28 days in common years.
ISO week 1 = the week containing the first Thursday of the ISO week-year
This is why dates near New Year's Day can belong to week 52 or 53 of the previous ISO year, or week 1 of the next ISO year.
Birthday weekday lookup
For the common search what day of the week was I born on, enter the birth date and read the headline weekday. The result also shows whether that birth date was a weekday or weekend, which day of the year it was, and which ISO week it belonged to.
Birthday weekday lookups are usually just for curiosity, but the same workflow is useful for anniversaries, memorial dates, school records, and family-history notes where a date may be remembered together with a weekday.
Future-date and planning checks
For future dates, the calculator works as a quick planning check. It can answer whether a renewal, event, appointment, campaign launch, payment date, or deadline falls on a Monday, Friday, weekend, or a specific ISO week.
The previous and next matching weekday are shown because the follow-up question is often practical: if the selected date is not the right weekday, what nearby date has the same weekday pattern? Use a business-days calculator instead when you need to count only working days or exclude public holidays.
ISO week number and week-year boundaries
ISO week numbers are not always the same as simple calendar-week labels. Weeks run Monday to Sunday, and week 1 is tied to the first Thursday of the ISO week-year. As a result, late December dates can belong to week 1 of the following ISO year, while early January dates can belong to the previous ISO year.
The calculator shows the ISO week-year and the Monday-to-Sunday week span so the answer is clear when a spreadsheet, payroll schedule, shipping plan, school term, or international record uses week numbers instead of month names.
Historical calendar caveat
This page uses the proleptic Gregorian calendar: the modern Gregorian rules are extended backward for older dates. That is useful for consistent calculation, but it may not match historical records from places and periods that used the Julian calendar or switched calendars at different times.
For archival, genealogical, legal, or academic work involving old records, confirm whether the source date is Julian, Gregorian, local civil time, or another calendar system before relying on the weekday alone.
Worked example: 2024-12-30
Enter 2024-12-30 and the calculator returns Monday. The date is day 365 of a leap year, so one day remains in the Gregorian calendar year after it.
The ISO context is the interesting part: 2024-12-30 is in ISO week 1 of ISO week-year 2025. That makes it a useful example for checking payroll, reporting, and planner systems that group dates by ISO week rather than by calendar month.
Frequently asked questions
What day of the week was I born on?
Enter your birth date in the date field. The calculator returns the weekday name, ISO day number, day of year, ISO week number, weekend status, and nearby matching weekdays for that birth date.
What day of the week is any date?
Enter the date in YYYY-MM-DD format or use the date picker. The result shows the weekday from Monday through Sunday and adds calendar facts that help verify the result.
Can I find the weekday for a future date?
Yes. Future dates use the same Gregorian calendar rules as current dates, so you can check deadlines, anniversaries, event dates, and planning dates in advance.
Does the calculator handle leap years and February 29?
Yes. Leap-year rules are part of the Gregorian calendar calculation. February 29 is accepted in valid leap years and rejected in common years.
What is the ISO week number?
The ISO week number identifies the Monday-to-Sunday week containing the date. ISO week 1 is the week containing the first Thursday of the ISO week-year, so dates around New Year's Day can belong to a different ISO year than their calendar year.
Why can December dates belong to ISO week 1 of the next year?
Because ISO weeks are based on whole Monday-to-Sunday weeks and the first Thursday rule. For example, a late-December Monday can start week 1 of the next ISO week-year.
Does the calculator use my local time zone?
No. The entered value is treated as a calendar date, not a moment in local time. That avoids timezone shifts that could otherwise move the date to the previous or next day.
Does it work for historical dates before 1582?
It returns a proleptic Gregorian result. That is consistent for calculation, but some historical records used the Julian calendar or changed calendars later than 1582, so archival work may need a calendar-conversion check.
What does ISO day number mean?
ISO day number labels Monday as 1 and Sunday as 7. It is useful when a spreadsheet, week-numbering system, or international record treats Monday as the first day of the week.
Can I use this as a business-day calculator?
No. This page identifies the weekday and date facts for one date. Use a business-days calculator when you need to count workdays, weekends, public holidays, or service-level deadlines between two dates.
