Day of the Week Calculator
Enter any date to instantly find out what day of the week it falls on. This calculator works for historical, current, and future dates, and also shows the day of the year and ISO week number.
Result
Saturday
March 28, 2026
Frequently Asked Questions
What day of the week was January 1, 2000?
January 1, 2000 was a Saturday. You can verify this with the Day of the Week Calculator by entering 2000-01-01 in the date field.
What day of the week is July 4, 2026?
July 4, 2026 is a Saturday. Independence Day falling on a Saturday means the US federal holiday is observed on Friday, July 3, 2026.
How do I find out what day of the week a date is?
Enter the date into our Day of the Week Calculator — it instantly shows the weekday name, whether the date is a weekend or weekday, the day of the year (1–365/366), and the ISO week number. For manual calculation, you can use Zeller's Congruence or the Doomsday Algorithm.
What is an ISO week number?
An ISO week number (ISO 8601) identifies a specific week within the year. Weeks run Monday to Sunday, and Week 1 is defined as the week containing the year's first Thursday. A year has either 52 or 53 ISO weeks. Days in early January may belong to the last week of the previous year.
What is the day of the year?
The day of the year (also called ordinal date) is a number from 1 to 365 (or 366 in leap years) counting from January 1. For example, February 1 is day 32, and December 31 is day 365 in a common year.
How do I calculate the day of the week manually?
Use Zeller's Congruence: h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7, where q is the day, m is the month (January and February are months 13 and 14 of the prior year), K is the year of the century, and J is the century. The result maps to 0=Saturday, 1=Sunday, 2=Monday, etc.
What day of the week does Christmas fall on in 2026?
Christmas (December 25, 2026) falls on a Friday. You can check any future or past date using our Day of the Week Calculator.
Can this calculator find historical dates before 1970?
Yes. The calculator supports any date the browser's date picker allows, typically from year 0001 through year 9999. It uses the proleptic Gregorian calendar, which extends the Gregorian calendar system backward before 1582 — the standard approach for historical date calculations.
What is the Day of the Week Calculator?
The Day of the Week Calculator instantly tells you what day of the week any date falls on — past, present, or future. Enter any date and the calculator returns the weekday name, whether it is a weekend or weekday, the day of the year (1–366), and the ISO week number. This is useful for scheduling, historical research, trivia, and any situation where you need to know the weekday for a specific date without consulting a paper calendar.
How to Find the Day of the Week for Any Date
Use the date picker at the top of the tool, or type a date directly into the input field. The calculator updates instantly as you change the date. To return to today's date, click the calendar icon next to the input.
- Click the date field and select or type the desired date.
- The large display shows the weekday name immediately.
- Below the large display, view the day index, day of year, and ISO week number.
- Click "Copy" to copy the full result to your clipboard.
How to Calculate the Day of the Week — Zeller's Congruence
Zeller's Congruence is a classic algorithm for computing the day of the week for any date in the Gregorian calendar. The formula for the Gregorian calendar is:
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ - 2J) mod 7
Where:
- h — the day of the week (0 = Saturday, 1 = Sunday, 2 = Monday, …, 6 = Friday)
- q — the day of the month
- m — the month (3 = March, 4 = April, …, 14 = February; January and February are counted as months 13 and 14 of the previous year)
- K — the year of the century (year % 100)
- J — the zero-based century (⌊year / 100⌋)
For example, to find the day for July 4, 2026: q = 4, m = 7, K = 26, J = 20. Plugging in gives h = 0, which maps to Saturday — the correct answer.
Our calculator uses JavaScript's built-in Date object which implements the same Gregorian calendar arithmetic, producing accurate results for any date from year 1 through year 9999.
Understanding ISO Week Numbers
The ISO 8601 week number is the internationally standardised way to refer to a specific week within a year. Key rules:
- Weeks run Monday to Sunday.
- Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4th).
- A year has either 52 or 53 ISO weeks. Years that start on Thursday, or leap years that start on Wednesday, have 53 weeks.
- Days in early January can belong to ISO Week 52 or 53 of the previous year, and days in late December can belong to Week 1 of the next year.
For example, January 1, 2026 is a Thursday, so it falls in ISO Week 1 of 2026. But January 1, 2016 was a Friday; since the first Thursday of 2016 was January 7th, January 1 belongs to ISO Week 53 of 2015.
Historical Calendar Facts
The Gregorian Calendar Reform (1582)
Pope Gregory XIII introduced the Gregorian calendar in October 1582 to correct accumulated drift in the Julian calendar. Ten days were skipped — October 4 was followed by October 15. Different countries adopted the reform at different times: Britain and its colonies switched in 1752, Russia not until 1918. This calculator uses the proleptic Gregorian calendar (extending backward before 1582), which is the standard for historical date calculations in most software.
The Seven-Day Week
The seven-day week has been in continuous use for over 2,000 years, tracing back to Babylonian astronomy and Jewish tradition. It was officially adopted by the Roman Empire in 321 AD when Emperor Constantine declared Sunday the first day of the week. The names of the days derive from classical planets and Norse gods: Sunday (Sun), Monday (Moon), Tuesday (Tiw/Mars), Wednesday (Woden/Mercury), Thursday (Thor/Jupiter), Friday (Frigg/Venus), and Saturday (Saturn).
Doomsday Algorithm
John Conway developed the Doomsday algorithmas a mental arithmetic trick to find the day of the week for any date. The algorithm exploits the fact that certain easy-to-remember dates always fall on the same day of the week within any given year — called the "Doomsday" for that year. For 2026, the Doomsday is Saturday. Memorizing a few anchor dates per century allows a trained person to calculate any day in their head within seconds.
Notable Historical Dates
| Date | Day | Event |
|---|---|---|
| July 4, 1776 | Thursday | US Declaration of Independence |
| November 11, 1918 | Monday | World War I armistice signed |
| July 20, 1969 | Sunday | Apollo 11 moon landing |
| January 1, 2000 | Saturday | Y2K / New Millennium |
| September 11, 2001 | Tuesday | 9/11 attacks |
| January 20, 2025 | Monday | US Presidential inauguration |
Day of the Year Reference
The "day of year" counts from January 1 (day 1) to December 31 (day 365, or day 366 in a leap year). This is sometimes called the ordinal date. Quick reference for the first day of each month in a non-leap year:
| Month | First Day of Month (day of year) |
|---|---|
| January | Day 1 |
| February | Day 32 |
| March | Day 60 |
| April | Day 91 |
| May | Day 121 |
| June | Day 152 |
| July | Day 182 |
| August | Day 213 |
| September | Day 244 |
| October | Day 274 |
| November | Day 305 |
| December | Day 335 |
In leap years, add 1 to all days from March 1 onwards.
Related Tools
- Date Calculator — Calculate the number of days between two dates or add/subtract days from a date
- Business Days Calculator — Count working days between two dates, excluding weekends
- Age Calculator — Calculate exact age in years, months, and days from a birthdate
- Countdown Calculator — Count down to a future event or date
Common Use Cases
- Verify the day of a historical event or birthday
- Determine if a future date falls on a weekend before scheduling
- Settle trivia disputes about what day a famous date occurred
- Check ISO week numbers for payroll, project management, or reporting
- Calculate the ordinal day of the year for scientific or logistical purposes
- Plan recurring weekly events by knowing what day a specific date lands on