Day of Week Calculator | What Day Was Any Date?
Find the day of the week for any date, past, present, or future. Shows ISO week number, day of year, quarter, leap year status, zodiac sign, and a mini calendar view. Includes famous historical dates and day-offset shortcuts.
Famous historical dates
Enter any date above, or tap a famous date to get started.
What Is the Day of Week Calculator | What Day Was Any Date??
Finding the day of the week for any arbitrary date requires a systematic algorithm because our calendar is not a simple 7-day repeating cycle, it accounts for months of varying length, leap years, and century adjustments.
Zeller's congruence (published by Christian Zeller in 1882) is the most widely cited formula. It works by encoding the date into a modular arithmetic expression that naturally produces a number from 0–6 representing the day of the week. January and February are treated as months 13 and 14 of the previous year to simplify the formula's month-length corrections.
The ISO 8601 week number is widely used in business and logistics. Week 1 is defined as the week containing the year's first Thursday (equivalently, the week containing January 4). This means a year always has either 52 or 53 ISO weeks, and late December days can belong to the following year's Week 1.
Formula
Zeller's Congruence
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7
Where:
h = day of week (0=Sat, 1=Sun, 2=Mon, … 6=Fri)
q = day of month
m = month (3=Mar, 4=Apr, … 14=Feb; Jan/Feb use previous year)
K = year within century (year % 100)
J = zero-based century (⌊year / 100⌋)
ISO 8601 Week Number
Step 1: Find the Thursday of the same ISO week as the date
Thursday = date + (4 − weekday) [Mon=1 … Sun=7]
Step 2: Count weeks from Jan 1 of Thursday's year
ISO week = ⌈(dayOfYear of Thursday) / 7⌉
Note: Week 1 always contains January's first Thursday.
Dec 29–31 may belong to Week 1 of the following year.
Jan 1–3 may belong to Week 52 or 53 of the previous year.
| Result field | Definition | Example (Jul 20, 1969) |
|---|---|---|
| Day of week | Which of 7 weekdays the date falls on | Sunday |
| ISO week | ISO 8601 week number (Mon–Sun, W01–W53) | W29 of 1969 |
| Day of year | Ordinal day count from Jan 1 = Day 1 | Day 201 |
| Quarter | Business quarter (Q1=Jan–Mar … Q4=Oct–Dec) | Q3 |
| Leap year | Whether the year has 366 days | No (365 days) |
| Zodiac sign | Western astrological sign for the date | ♋ Cancer |
How to Use
- 1Pick a date: Type or click to select any date in the input field. Results appear instantly, no button needed.
- 2Try a famous date: Click one of the preset buttons (Moon Landing, D-Day, etc.) to explore historical dates.
- 3Adjust by days: Use the ±1d / ±7d / ±30d buttons to step forward or backward from the current date.
- 4Read the results: See the full day name, ISO week number, day of year, quarter, leap year, and zodiac sign.
- 5Check the mini calendar: The calendar highlights the selected date in its month for quick visual context.
- 6Reset: Click Reset or press Esc to clear the input and start fresh.
Example Calculation
Example 1, Apollo 11 Moon Landing (July 20, 1969)
- ›Date: July 20, 1969.
- ›Day of week: Sunday, Neil Armstrong stepped onto the lunar surface on a Sunday.
- ›ISO week: W29 of 1969. Day of year: 201.
- ›Quarter: Q3 (July–September). Not a leap year (365 days).
- ›Zodiac: Cancer (Jun 21 – Jul 22).
Example 2, ISO Week Edge Case (January 1, 2016)
- ›January 1, 2016 was a Friday.
- ›ISO week: W53 of 2015, not Week 1 of 2016!
- ›This happens because the first Thursday of 2016 was January 7 (Week 1).
- ›So Jan 1–3 of 2016 belong to the last ISO week of 2015.
- ›This is why the ISO year shown can differ from the calendar year for early January dates.
Example 3, Planning a Recurring Meeting
- ›You need to schedule a quarterly review on the first Monday of each quarter.
- ›Enter January 1, 2025 → Tuesday. First Monday = January 6, 2025.
- ›Enter April 1, 2025 → Tuesday. First Monday = April 7, 2025.
- ›Enter July 1, 2025 → Tuesday. First Monday = July 7, 2025.
- ›Use the +7d adjustment button to step through weeks to find the right date quickly.
Understanding Day of Week | What Day Was Any Date?
Why Knowing the Day of the Week Matters
Calendar calculations are surprisingly common in professional and everyday contexts. Project managers need to know whether a deadline falls on a weekend. HR teams check if a contract start date is a Monday. Legal documents often specify that a notice period starts on the next business day. Historians want to know what day famous events fell on. And anyone planning an event needs to know which day of the week a future date lands.
Without a reliable tool, these calculations require counting days on paper or flipping through a physical calendar, tedious and error-prone. This calculator handles the arithmetic instantly, including edge cases like century years, ISO week boundaries, and leap year adjustments.
The Doomsday Algorithm, Mental Math for Weekdays
John Conway (creator of the Game of Life) devised the Doomsday algorithm for calculating the day of the week mentally. The key insight is that certain easily-remembered dates always fall on the same weekday within any given year, he called this the “Doomsday”:
- ›These dates always share the same weekday in any year: Jan 3/4, Feb 28/29, Apr 4, Jun 6, Aug 8, Oct 10, Dec 12.
- ›Also: the last day of Feb, 5/9, 9/5, 7/11, and 11/7 ("I work 9 to 5 at 7-11").
- ›Find the Doomsday for the year, then count from the nearest anchor date to your target.
- ›Example: 2025's Doomsday is Friday. Dec 12 is a Friday. Dec 25 = Dec 12 + 13 days = Thursday.
- ›With practice, most people can name any date's weekday in under 10 seconds.
Famous Dates and Their Weekdays
| Event | Date | Day |
|---|---|---|
| Apollo 11 moon landing | July 20, 1969 | Sunday |
| D-Day (Normandy landings) | June 6, 1944 | Tuesday |
| Fall of the Berlin Wall | November 9, 1989 | Thursday |
| 9/11 attacks | September 11, 2001 | Tuesday |
| New Year 2000 (Y2K) | January 1, 2000 | Saturday |
| JFK assassination | November 22, 1963 | Friday |
| Declaration of Independence | July 4, 1776 | Thursday |
| Christmas Day 2025 | December 25, 2025 | Thursday |
Frequently Asked Questions
What day of the week was I born?
- ›Enter your date of birth in the input field, the day of the week appears instantly.
- ›The calculator also shows your zodiac sign, what ISO week you were born in, and the day of that year.
- ›Monday's child is fair of face, the old nursery rhyme assigned personality traits to each birth weekday.
- ›For a full age breakdown (years, months, days, hours), try the Age Calculator.
How does Zeller's congruence work?
- ›Zeller's formula maps a Gregorian date to an integer 0–6 using modular arithmetic.
- ›January and February are treated as months 13–14 of the prior year to handle year-end leap day adjustments.
- ›The formula accounts for century corrections (÷4 of the century) that arise from the 400-year leap year cycle.
- ›The result h maps to days as: 0=Sat, 1=Sun, 2=Mon, 3=Tue, 4=Wed, 5=Thu, 6=Fri.
- ›Alternative algorithms include the Doomsday algorithm (used for mental math) and Tomohiko Sakamoto's one-liner.
What is an ISO week number?
- ›ISO 8601 defines weeks as starting on Monday and ending on Sunday.
- ›Week 1 (W01) is the week containing the first Thursday of the year.
- ›An equivalent rule: W01 is the week containing January 4th.
- ›Years have either 52 or 53 ISO weeks. Years with 53 weeks are called long years.
- ›ISO week notation: 2024-W52-3 means Wednesday, ISO week 52 of 2024.
- ›This system is widely used in European business scheduling and supply chain management.
Is the year 2100 a leap year?
- ›No, 2100 is not a leap year, despite being divisible by 4.
- ›The Gregorian leap year rule has three conditions: divisible by 4 (leap), UNLESS divisible by 100 (not leap), UNLESS divisible by 400 (leap again).
- ›Examples: 1900 not leap, 2000 leap (divisible by 400), 2100 not leap, 2400 will be leap.
- ›Without the century exception, the calendar would drift by about 0.75 days per century.
- ›With the full rule, the Gregorian calendar stays accurate to within 1 day over 3,236 years.
Why might the ISO year differ from the calendar year?
- ›ISO week years and calendar years can differ for days in late December or early January.
- ›If January 1 is a Friday, Saturday, or Sunday, it belongs to the last week of the previous ISO year.
- ›Example: Jan 1, 2016 (Friday) → ISO week 53 of 2015, not week 1 of 2016.
- ›Similarly, Dec 29–31 can belong to week 1 of the following ISO year.
- ›This is not an error, it ensures every ISO week is a complete Mon–Sun week.
How accurate is this calculator for very old dates?
- ›The calculator uses the proleptic Gregorian calendar, it projects the Gregorian rules backward before 1582.
- ›In reality, most of Europe used the Julian calendar until October 15, 1582 (when the Gregorian calendar was adopted).
- ›Different countries switched at different times: Britain switched in 1752; Russia in 1918.
- ›For dates before 1582, the day shown is what the Gregorian calendar would have said, not what people actually called that day at the time.
- ›For precise historical research, use a Julian-to-Gregorian conversion tool alongside this calculator.
What day of the week repeats every year on the same date?
- ›The day of the week for any given date shifts by 1 each common year and by 2 each leap year.
- ›For example, if January 1 is a Monday this year, it will be a Tuesday next year (assuming no leap year in between).
- ›The full pattern repeats every 28 years in the Gregorian calendar (the 28-year solar cycle).
- ›Within a year, some dates always share the same day of week: Jan 4, Apr 4, Jun 6, Aug 8, Oct 10, Dec 12, and the last day of Feb are all the same weekday in any given year (the Doomsday rule).
What are the quarters in a year?
- ›Q1: January 1 – March 31 (90 or 91 days depending on leap year).
- ›Q2: April 1 – June 30 (91 days).
- ›Q3: July 1 – September 30 (92 days).
- ›Q4: October 1 – December 31 (92 days).
- ›Fiscal quarters may differ from calendar quarters, e.g., the US federal fiscal year starts October 1.
- ›The calculator always shows the calendar quarter (Q1–Q4 based on the Gregorian calendar).