Weekday Calculator -- What Day Is That Date?

How Day-of-Week Calculation Works

Finding the day of the week for any date is possible using mathematical formulas. The most famous is Zeller's Congruence, an algorithm developed by Christian Zeller in the 19th century.

Zeller's Congruence Formula

For the Gregorian calendar, the formula is:

h = (q + floor(13(m+1)/5) + K + floor(K/4) + floor(J/4) - 2J) mod 7

Where:

• h is the day of the week (0 = Saturday, 1 = Sunday, 2 = Monday, ..., 6 = Friday)
• q is the day of the month
• m is the month (3 = March, 4 = April, ..., 14 = February; January and February are counted as months 13 and 14 of the previous year)
• K is the year of the century (year mod 100)
• J is the zero-based century (floor(year / 100))

Example Calculation

Find the day for December 25, 2026:

• q = 25
• m = 12 (December)
• K = 26 (2026 mod 100)
• J = 20 (floor(2026 / 100))

h = (25 + floor(13*13/5) + 26 + floor(26/4) + floor(20/4) - 2*20) mod 7
h = (25 + 33 + 26 + 6 + 5 - 40) mod 7
h = 55 mod 7 = 6 (Friday)

Result: December 25, 2026 falls on a Friday.

Leap Year Rules

A year is a leap year if it is divisible by 4, except for century years which must be divisible by 400.

Leap years have 366 days (with February 29) instead of the usual 365. This compensates for the fact that Earth's orbit around the Sun takes approximately 365.2425 days.

Understanding ISO Week Numbers

The ISO 8601 standard defines a week numbering system used internationally in business and government:

• Weeks start on Monday and end on Sunday
• Week 1 of a year is the week containing the first Thursday of January
• Equivalently, Week 1 is the week containing January 4
• A year has either 52 or 53 weeks

This means some dates in early January may belong to the last week of the previous year, and some dates in late December may belong to Week 1 of the next year.

Example

January 1, 2026 is a Thursday. Since it is the first Thursday of the year, it falls in Week 1. But January 1, 2027 is a Friday, so the week containing it (starting Monday, December 29, 2026) is still Week 53 of 2026.

US Federal Holidays

This calculator checks for the following US federal holidays:

Frequently Asked Questions

How does this weekday calculator work?

This calculator uses the built-in JavaScript Date object to determine the day of the week for any date. It works for dates far in the past and future. The Gregorian calendar is used for all calculations.

What is the ISO week number?

The ISO week number follows the ISO 8601 standard. Week 1 is the week containing the first Thursday of the year. Weeks start on Monday. A year has 52 or 53 weeks. The ISO week date system is widely used in business and international contexts.

What holidays does this tool check?

This tool checks for major US federal holidays including New Year's Day, Martin Luther King Jr. Day, Presidents Day, Memorial Day, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving, and Christmas. Some holidays fall on fixed dates while others are relative (like the third Monday of January).

Can I check dates far in the past or future?

Yes. JavaScript Date objects support dates from approximately 271,821 BCE to 275,760 CE. You can check what day of the week any historical or future date falls on.

Why do some years have 53 weeks?

In the ISO 8601 week numbering system, a year has 53 weeks when January 1 falls on a Thursday or when it is a leap year and January 1 falls on a Wednesday. This happens approximately every 5-6 years.

What is the day of year?

The day of year is the sequential day number within a calendar year. January 1 is day 1, December 31 is day 365 (or 366 in a leap year). It is useful for tracking progress through the year and for scientific calculations.

Does this calculator account for time zones?

No. This calculator works with calendar dates only, not times. The day of the week for a specific date is the same regardless of time zone.

Is this tool accurate for historical dates before 1582?

The calculator uses the Gregorian calendar, which was adopted in October 1582. For dates before that, most of the world used the Julian calendar with different leap year rules. Results for pre-1582 dates are based on the proleptic Gregorian calendar (extending the current calendar backward).

Related Tools

Privacy & Limitations

• Client-side only. All calculations run in your browser. No data is sent to any server.
• Gregorian calendar. Uses the Gregorian calendar system. Results for dates before October 15, 1582 are based on the proleptic Gregorian calendar.
• US holidays only. Holiday detection covers US federal holidays. State, local, and international holidays are not included.
• No time zones. Calculations use calendar dates only. The day of the week is the same regardless of time zone.

Weekday Calculator FAQ