Enter your birth date and we'll tell you the exact weekday.

How accurate is the Day of the Week?

It uses straightforward, exact formulas, so the result is reliable for the values you enter.

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Day of the Week

Find the weekday, day of year and week number for any date.

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Date

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Day of the week

Thursday

September 17, 2026

Day of year

260

Week number

AI Breakdown & Smart Takeaway

Plain-English insight on your numbers

Get a personalized explanation of what these results mean — and how to improve them.

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How the Day of the Week works

The Day of the Week calculator instantly tells you the weekday name, day of the year (1–366), and ISO week number for any date you enter — past, present, or future. It's perfect for anyone who needs to know what day they were born, plan events, verify historical dates, or satisfy everyday curiosity.

At its core, this calculator converts a calendar date (year, month, day) into a weekday by applying a modular arithmetic algorithm to the continuous count of days since a fixed reference point. The most widely used reference is the Julian Day Number, but simpler implementations use the Gregorian calendar epoch directly. The result of dividing the total elapsed days by 7 gives a remainder (0–6) that maps precisely to Monday through Sunday or Sunday through Saturday depending on the convention chosen.

The 'day of year' value (also called the ordinal date) is calculated by summing the days in all preceding months plus the current day number. This requires knowing whether the year is a leap year, since February has 29 days instead of 28 in years divisible by 4 — except century years, which must also be divisible by 400 to qualify. For example, 2000 was a leap year but 1900 was not, which affects every date calculation in those years.

Week numbers follow the ISO 8601 standard, where weeks run Monday to Sunday and Week 1 is defined as the week containing the year's first Thursday. This means January 1 can fall in Week 52 or 53 of the previous year, and December 31 can fall in Week 1 of the next year — a common source of confusion. Some software and regions (particularly the US) use a 'Week 1 = the week containing January 1' convention instead, so it is worth knowing which system applies to your context.

A common mistake is forgetting that the Gregorian calendar was adopted at different times in different countries, so historical dates before the mid-1700s may appear on a different weekday depending on whether you use the Julian or Gregorian system. The calculator uses the proleptic Gregorian calendar for all dates, meaning it applies today's calendar rules backwards uniformly — which is the standard for modern computation but may differ from contemporaneous historical records.

Formula

Tomohiko Sakamoto's algorithm (or Zeller's Congruence): h = (q + floor(13(m+1)/5) + K + floor(K/4) + floor(J/4) - 2J) mod 7, where q = day, m = month (3=March…14=February), K = year-of-century, J = zero-based century. Day of Year = sum of days in months 1 to (m-1) + day + leap-day adjustment.

Pro tips

When finding the weekday for a birth date, double-check whether the year was a leap year if the date is February 28 or 29 — this single detail changes every day-of-year calculation for dates after February in that year.
Use the ISO week number to align with international business schedules, project timelines, or European calendars; but switch to the US week convention (Week 1 = week of Jan 1) when working with US payroll or fiscal systems to avoid off-by-one errors.
To mentally verify any calculator result, remember that January 1, 2000 was a Saturday — you can count forward or backward in blocks of 7 (weeks) or use the fact that a regular year shifts the weekday by 1 day and a leap year shifts it by 2.
Day-of-year values are especially useful in astronomy, logistics, and agriculture, where professionals refer to 'Julian dates' or 'DOY' rather than month/day — knowing your ordinal date lets you communicate instantly across these fields.
If you're researching a historical event before October 15, 1582 (the Gregorian reform date), be aware that the calculator's proleptic Gregorian result may differ by 10–13 days from what contemporaries actually recorded under the Julian calendar.

Key terms

Weekday: — The named day of the seven-day week (Monday through Sunday) on which a specific calendar date falls.
Ordinal Date (Day of Year): — A number from 1 to 365 (or 366 in a leap year) representing a date's position within its calendar year, counting from January 1 as day 1.
ISO Week Number: — A standardized week count (ISO 8601) where Week 1 contains the year's first Thursday, weeks run Monday–Sunday, and a year has either 52 or 53 weeks.
Leap Year: — A year with 366 days in which February has 29 days, occurring in years divisible by 4, except century years must also be divisible by 400.
Proleptic Gregorian Calendar: — The extension of the Gregorian calendar system backward to dates before its historical adoption, used as the standard for uniform date calculations.
Zeller's Congruence: — A classic mathematical algorithm that computes the weekday of any Gregorian or Julian date using modular arithmetic.