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Day of the Week Calculator — Find Any Day Instantly

Discover which day of the week any date falls on, whether it is a historical date, today, or a future date, with ISO day numbering and weekday-weekend classification.

Day of the Week

Wednesday

February 25, 2026

ISO Day Number

3

Type

Weekday

ISO 8601 Standard

Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6, Sunday = 7

How to Use the Day of the Week Calculator

  1. Select any date: Use the date picker to choose the date you want to investigate. The input defaults to today's date. You can navigate to any year and month using the calendar controls, or type the date directly in YYYY-MM-DD format. The calculator accepts dates far into the past and future.
  2. View the day name: The result panel immediately displays the day of the week in large, prominent text. The full date is shown below for confirmation. This tells you at a glance whether the date is a Monday, Tuesday, or any other day.
  3. Check the ISO day number: The ISO 8601 day number is shown alongside the day name. This is useful for programmers, data analysts, and anyone working with international date standards. Monday is 1, Tuesday is 2, continuing through Sunday which is 7.
  4. See weekday or weekend status: A color-coded badge indicates whether the date falls on a weekday (green) or weekend (amber). This is helpful for planning events, checking holiday schedules, or verifying business day availability.

The calculator updates instantly when you change the date, making it easy to explore multiple dates in quick succession. Try checking birthdays, anniversaries, historical events, or future holidays to discover which day of the week they fall on.

Day of the Week Formula

Day = (JDN mod 7) mapped to {Mon=1, Tue=2, ..., Sun=7}
Zeller: h = (q + floor(13(m+1)/5) + K + floor(K/4) + floor(J/4) - 2J) mod 7

Variables Explained

  • JDN (Julian Day Number): A continuous count of days since the beginning of the Julian Period (January 1, 4713 BC in the Julian calendar). By taking this number modulo 7, we can determine the day of the week for any date.
  • q: The day of the month (1-31).
  • m: The month number in Zeller's convention where March = 3, April = 4, ..., December = 12, January = 13, February = 14 (January and February are treated as months 13 and 14 of the previous year).
  • K: The year of the century (year mod 100).
  • J: The zero-based century (floor of year / 100).
  • h: The resulting day of the week where 0 = Saturday, 1 = Sunday, 2 = Monday, ..., 6 = Friday in Zeller's convention.

Step-by-Step Example

Find the day of the week for July 4, 2026 (Independence Day):

  1. Using JavaScript Date: new Date(2026, 6, 4).getDay() returns 6
  2. JavaScript day 6 = Saturday (0=Sun, 1=Mon, ..., 6=Sat)
  3. ISO day number: Saturday = 6
  4. Result: July 4, 2026 is a Saturday
  5. Classification: Weekend day
  6. This means Independence Day 2026 naturally falls on a weekend

With Zeller's formula: q=4, m=7, K=26, J=20. h = (4 + floor(13*8/5) + 26 + floor(26/4) + floor(20/4) - 40) mod 7 = (4 + 20 + 26 + 6 + 5 - 40) mod 7 = 21 mod 7 = 0, which maps to Saturday in Zeller's convention.

Practical Examples

Example 1: Lisa's Birthday Party Planning

Lisa was born on August 15, 1995 and wants to know what day she was born on. She also wants to check which day her birthday falls on in 2026 to plan a celebration:

  • August 15, 1995: Tuesday
  • August 15, 2026: Saturday
  • This means her 2026 birthday falls on a weekend — perfect for a party

Lisa discovers she was born on a Tuesday ("Tuesday's child is full of grace" according to the nursery rhyme). Since her 2026 birthday lands on a Saturday, she can plan a full day celebration without worrying about work schedules. She uses the what day calculator to check several future years and finds that her birthday falls on a Saturday again in 2032.

Example 2: Nathan's Historical Research

Nathan is a history student researching the day of the week for significant events. He checks several dates:

  • July 20, 1969 (Moon landing): Sunday
  • November 9, 1989 (Berlin Wall fall): Thursday
  • September 11, 2001: Tuesday
  • January 20, 2009 (Obama inauguration): Tuesday

Nathan notes that the Moon landing occurred on a Sunday, which meant most Americans were home and could watch it live on television — a factor that contributed to the event's massive viewership. The Berlin Wall fell on a Thursday evening, leading to a weekend of celebration. Understanding the day of the week adds context to historical events and helps explain public reaction and media coverage patterns.

Example 3: Priya's Meeting Scheduler

Priya needs to schedule a quarterly board meeting on the 15th of every month in 2026 and needs to know which months have the 15th falling on a weekday:

  • January 15: Thursday (weekday)
  • February 15: Sunday (weekend)
  • March 15: Sunday (weekend)
  • April 15: Wednesday (weekday)
  • July 15: Wednesday (weekday)
  • October 15: Thursday (weekday)

Priya discovers that February and March have the 15th on a Sunday, so she needs to move those meetings to the 16th (Monday) or the 13th (Friday). For checking the corresponding time zones of remote attendees, the time zone calculator helps coordinate across international teams.

Days of the Week Reference Table

Day Name ISO Number JS Number Type Origin
Monday 1 1 Weekday Moon's day
Tuesday 2 2 Weekday Tiw's day (Norse god)
Wednesday 3 3 Weekday Woden's day (Norse god)
Thursday 4 4 Weekday Thor's day (Norse god)
Friday 5 5 Weekday Freya's day (Norse goddess)
Saturday 6 6 Weekend Saturn's day (Roman god)
Sunday 7 0 Weekend Sun's day

Tips and Complete Guide

The Doomsday Algorithm

Mathematician John Conway developed the "Doomsday algorithm" as a mental shortcut for calculating the day of the week. The key insight is that certain easy-to-remember dates always fall on the same day of the week in any given year (called the "Doomsday"). These anchor dates include 4/4, 6/6, 8/8, 10/10, 12/12, the last day of February, and 7/11 and 11/7. Once you know the Doomsday for a year, you can count forward or backward from the nearest anchor date. For 2026, the Doomsday is Saturday. This means April 4, June 6, August 8, October 10, and December 12 all fall on Saturday in 2026.

Calendar Patterns and Cycles

The Gregorian calendar repeats on a 400-year cycle containing exactly 146,097 days (20,871 weeks). Within this cycle, certain patterns emerge: the 14 possible calendar configurations (7 possible starting days times 2 for leap/common years) rotate in a predictable pattern. A common year shifts the starting day by 1, and a leap year shifts it by 2. The calendar for 2026 is identical to the calendars for 2015, 2009, 1998, and 1987. Understanding these cycles can help you quickly determine day-of-week patterns for groups of years.

Practical Applications Beyond Curiosity

Knowing the day of the week has practical applications in many fields. Financial markets operate Monday through Friday — knowing which day a date falls on determines whether trading occurs. Religious observances are tied to specific days: Christian worship on Sunday, Jewish Shabbat from Friday evening to Saturday evening, Islamic Jumu'ah prayers on Friday. Delivery services, restaurant reservations, gym schedules, and medical offices all operate on weekly cycles. Event planners must know the day of the week to set appropriate pricing (weekend vs. weekday rates) and staffing levels.

Common Mistakes to Avoid

  • Confusing calendar systems: The calculator uses the Gregorian calendar. Dates before October 15, 1582 (when the Gregorian calendar was adopted) may not match historical records that used the Julian calendar. The difference between Julian and Gregorian calendars grows by about 3 days every 400 years.
  • Assuming regular patterns: While the day of the week advances by 1 each common year, leap years cause a 2-day shift for dates after February. Do not assume that because a date is Tuesday this year, it will be Wednesday next year — it depends on whether the intervening period includes a February 29.
  • Forgetting time zone effects: A date that is Monday in New York may still be Sunday in Honolulu or already Tuesday in Tokyo. When checking the day of the week for an event in a different time zone, make sure you are using the local date for that time zone.
  • Mixing up ISO and JavaScript day numbering: In ISO 8601, Monday = 1 and Sunday = 7. In JavaScript, Sunday = 0 and Saturday = 6. These two systems can cause confusion in programming contexts. Our calculator shows the ISO number for clarity.

Frequently Asked Questions

The calculator uses JavaScript's built-in Date object, which implements the Gregorian calendar proleptic system. When you select a date, the Date object internally computes the day of the week using algorithms based on the Julian Day Number. It returns a number from 0 (Sunday) to 6 (Saturday), which the calculator then maps to the full day name. The ISO 8601 standard numbers days differently: Monday = 1 through Sunday = 7. Our calculator shows both the day name and the ISO number for maximum utility.

Enter your date of birth into the calculator to instantly see which day of the week you were born on. Many cultures attach significance to birth days — the nursery rhyme 'Monday's child is fair of face' assigns personality traits based on birth day. Statistically, births in the United States are most common on Tuesdays and least common on Sundays, largely due to scheduled cesarean sections and induced deliveries being planned for weekdays. The calculator works for any date, so you can also check birth days for family members and historical figures.

ISO 8601 is an international standard for date and time representation. Under this standard, Monday is day 1 and Sunday is day 7, making the work week a consecutive run from day 1 to day 5. This differs from the JavaScript/US convention where Sunday is 0 and Saturday is 6. The ISO system is used in international business, computing, and data exchange. Our calculator displays the ISO day number alongside the day name so the result is useful regardless of which numbering convention you need.

Yes, the calculator works for any valid date — past, present, or future. This is useful for planning events, scheduling meetings, or checking which day of the week a holiday falls on. For example, you can check whether Christmas 2026 falls on a weekday or weekend (it falls on a Friday), or plan a birthday party by finding which day of the week your birthday falls on in a future year. The Gregorian calendar repeats on a 400-year cycle, so patterns eventually recur.

Week start conventions are cultural and religious in origin. The ISO 8601 standard designates Monday as the first day of the week, which is followed in most of Europe, Asia, and South America. The United States, Canada, Japan, and several other countries traditionally start the week on Sunday, influenced by Judeo-Christian tradition where Sunday is the first day. Some Middle Eastern countries start the week on Saturday. Our calculator shows the ISO day number (Monday = 1) as the international standard, while the day name itself is universally understood regardless of week-start convention.

The calculator uses the Gregorian calendar proleptic extension, meaning it projects the Gregorian calendar rules backward even before the calendar was officially adopted in October 1582. For dates before 1582, the calculator shows what the Gregorian equivalent would be, though historically those dates used the Julian calendar. For practical purposes, the calculator is most accurate for dates from 1582 onward. Very ancient dates may differ from historical records that used the Julian calendar or other calendar systems.

Zeller's congruence is a mathematical algorithm devised by Christian Zeller in 1887 to calculate the day of the week for any date. The formula is: h = (q + floor(13(m+1)/5) + K + floor(K/4) + floor(J/4) - 2J) mod 7, where q is the day of the month, m is the month (March=3 through February=14), K is the year of the century, and J is the century. While our calculator uses JavaScript's native date implementation rather than Zeller's formula directly, both methods produce identical results for Gregorian calendar dates.

A common (non-leap) year has 365 days, which equals 52 weeks plus 1 day. This means any given date shifts forward by one day of the week from one common year to the next. A leap year has 366 days (52 weeks plus 2 days), so dates after February in a leap year shift forward by two days compared to the previous year. For example, if January 1 is a Wednesday in a common year, it will be Thursday the following year — unless that following year is a leap year, in which case dates from March onward shift by an extra day.

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Disclaimer: This calculator is for informational and educational purposes only. Results are estimates and may not reflect exact values.

Last updated: February 23, 2026

Sources