About this tool
The Day of the Week Calculator tells you what day (Monday-Sunday) any date falls on - past or future. Uses the Gregorian calendar. Great for planning events and satisfying curiosity.
About the day-of-week calculator
The day-of-week calculator answers the simple question: what weekday does a given date fall on? It works for any date from the early Common Era forward and uses the proleptic Gregorian calendar (the modern rules extended back through history) so the answer matches how almost every modern computer, government, and calendar app interprets historical dates.
The use cases run from genealogy ("what day was my grandfather born?") through legal calendar work (deadlines that fall on weekends), event planning (anniversaries, project go-lives), and contract calculations (whether the 30-day cure period ends on a business day). It is also a sanity check for spreadsheet date arithmetic, where Excel and Google Sheets occasionally drift over century boundaries.
How it works
// JavaScript Date object (proleptic Gregorian)
new Date("2026-05-28T12:00:00").getDay() // 0=Sun, 1=Mon, ... 6=Sat
// Zeller's congruence (paper formula, post-1582 Gregorian):
// q = day, m = month (Mar=3 ... Dec=12, Jan/Feb count as 13/14 of prev year)
// K = year mod 100, J = year div 100
h = (q + floor(13*(m+1)/5) + K + floor(K/4) + floor(J/4) - 2*J) mod 7
// h: 0=Sat, 1=Sun, 2=Mon, ... 6=Fri
Most calculators on the web (including this one) defer to the browser's Date object because it is exhaustively tested across decades of edge cases. Zeller's congruence and John Conway's 1973 Doomsday algorithm are the two formulas mathematicians use for pen-and-paper computation. Both give the same answer for any date from 15 October 1582 onward.
Worked example: 28 May 2026
Using Zeller's congruence: q=28, m=5 (May), K=26 (year mod 100), J=20 (year div 100).
- floor(13 * (5+1) / 5) = floor(78/5) = 15.
- floor(K/4) = floor(26/4) = 6.
- floor(J/4) = floor(20/4) = 5.
- Sum = 28 + 15 + 26 + 6 + 5 - 40 = 40.
- 40 mod 7 = 5. h=5 maps to Thursday in Zeller's encoding (0=Sat, 5=Thu).
Notable date facts
| Date | Day | Why it matters |
|---|---|---|
| 15 October 1582 | Friday | First Gregorian calendar date (replaced 5 October Julian) |
| 14 September 1752 | Thursday | Britain and colonies switched (jumped from 2 September) |
| 1 January 1900 | Monday | Common Excel epoch reference point |
| 1 January 2000 | Saturday | Millennium rollover (Y2K) |
| 29 February 2000 | Tuesday | Centurial leap year (divisible by 400) |
| 29 February 2100 | does not exist | Centurial non-leap (divisible by 100 only) |
| 1 January 2026 | Thursday | Current year reference |
Common pitfalls
- Confusing Julian and Gregorian for old dates. Shakespeare died on 23 April 1616 Julian, which is 3 May 1616 Gregorian. Historical references vary, so check the calendar convention before tagging a day-of-week.
- Forgetting the British 1752 switch. Anglo-American dates between 1582 and 1752 are Julian. Continental European dates in the same span are Gregorian.
- Spreadsheet 1900 leap-year bug. Excel incorrectly treats 1900 as a leap year for legacy compatibility with Lotus 1-2-3. Dates before 1 March 1900 in Excel are off by one day from the true Gregorian day-of-week.
- Time-zone shift. Entering an ISO date like 2026-05-28 in JavaScript defaults to UTC midnight, which can roll back to the previous day in negative time zones. The calculator above anchors the time to noon to dodge this.
- Birth year vs conception year. When someone says "I was born in 2000" they may mean January 2000 (Saturday) or December 2000 (the day-of-week will differ). Always confirm the exact date.
Related time and date tools
Frequently asked questions
How do I work out what day of the week any date is?
This calculator uses JavaScript's built-in Date object, which applies the proleptic Gregorian calendar (Gregorian rules extended back through history). For paper-and-pencil work, Zeller's congruence and the Doomsday algorithm (John Conway, 1973) both give the answer in under 30 seconds with practice. The formula is: take year code, add month code, add day, modulo 7. The result maps to Sunday through Saturday.
Why do some days appear to repeat on the same date every 28 years?
Because the Gregorian calendar's week-day pattern cycles every 28 years for ordinary years and every 400 years overall (the full cycle that respects all leap-year rules). That is why a calendar from 1998 works for 2026 if neither year crosses a century boundary that breaks the simple 28-year cycle. The repeat is not exact across century boundaries (1900 was not a leap year, 2000 was).
What is the difference between Gregorian and Julian dates?
The Julian calendar (introduced 45 BCE by Julius Caesar) had a leap year every 4 years with no exceptions, producing an average year of 365.25 days. The Gregorian calendar (introduced 1582 by Pope Gregory XIII) drops the leap year in centurial years not divisible by 400, giving 365.2425 days. By 1582 the Julian had drifted 10 days; the switch jumped 4 October to 15 October. Britain and its colonies (including America) did not switch until 1752.
What day of the week was I born?
Enter your birth date in the form above. As a sanity check, here are a few common years: 1 January 1980 was a Tuesday, 1 January 1990 was a Monday, 1 January 2000 was a Saturday, 1 January 2010 was a Friday, 1 January 2020 was a Wednesday. The calculator handles any date from 100 CE onward in modern browsers.
Does this work for dates before 1582?
Technically yes, but with a caveat. The calculator uses the proleptic Gregorian calendar (Gregorian rules extended backward), so it returns the Gregorian day-of-week even for dates in the Julian era. Historians who care about the actual day a Roman, medieval, or early modern document was written should look up the Julian-Gregorian conversion separately. For genealogy after 1752 in English-speaking countries, the proleptic Gregorian is correct.