Day of the Week Calculator
Enter any date to instantly see its day of the week, plus the day number of the year — calculated in your browser.
Wednesday, June 10, 2026
Wednesday
Day of Year
161 / 365
Leap Year
No
How to use Day of the Week Calculator
The Day of the Week Calculator tells you exactly which weekday any date falls on — past, present, or future. Enter a birth date, a historical event, or a future appointment to instantly see whether it lands on a Monday, Friday, or any other day, along with the day-of-year number (1–365 or 1–366 in a leap year). All computation runs locally in your browser using the proleptic Gregorian calendar, so your data is never uploaded anywhere.
- Select or type a date in the date picker — it defaults to today.
- The weekday name appears instantly below the input.
- Check the "Day of Year" card to see which day of the year it is (e.g. Day 1 on January 1, Day 365 on December 31 in a non-leap year).
- Use the "Leap Year" card to see whether the year has 365 or 366 days.
- Click "Copy result" to copy the full formatted answer for use in documents or messages.
Your data never leaves your device — 100% private processing.
How weekday calculation works
The algorithm maps a calendar date to its weekday by computing the number of days elapsed since a known anchor (for example, January 1, 2000 was a Saturday) and taking that count modulo 7. This is mathematically equivalent to Zeller's congruence, a formula developed by Christian Zeller in 1882. JavaScript's built-in Date object performs this calculation natively, accounting for all leap-year corrections in the Gregorian calendar. The result is reliable for thousands of years in either direction.
Leap years and the day-of-year count
A leap year contains 366 days instead of the usual 365 — February gains an extra day (the 29th). A year is a leap year if it is divisible by 4, except for century years (divisible by 100), which must also be divisible by 400. So 2000 and 2400 are leap years, but 1900 and 2100 are not. The day-of-year counter resets on January 1 (Day 1) and reaches 365 or 366 on December 31. Knowing the day-of-year is useful for comparing progress through the year, planning annual events, and certain financial day-count conventions.
Practical uses for the weekday calculator
Common uses include: finding what day of the week you were born on; checking whether a historical date like D-Day (June 6, 1944 — Tuesday) or the moon landing (July 21, 1969 — Monday) was a weekday or weekend; planning events by confirming a future date is not a weekend; verifying contract start dates; and satisfying everyday curiosity about important anniversaries. Historians, genealogists, and puzzle enthusiasts find it especially useful for matching diary entries and historical records to accurate weekdays.
Glossary
- Proleptic Gregorian calendar
- The Gregorian calendar extended backward before its 1582 adoption, used for consistent historical date arithmetic.
- Zeller's congruence
- A classic algorithm by Christian Zeller that computes the day of the week for any Gregorian or Julian calendar date.
- Day of year
- The ordinal position of a date within its year, from Day 1 (January 1) to Day 365 or 366 (December 31).
- Leap year
- A calendar year with 366 days — February 29 is added — occurring every 4 years with century-year exceptions.
- ISO 8601
- The international date format standard, written YYYY-MM-DD, which removes ambiguity between date formats across cultures.
Related reading
Frequently Asked Questions
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Why use Day of the Week Calculator?
- Handles timezone conversions and DST automatically
- Supports date arithmetic across years and leap years
- ISO 8601 and human-readable output formats
- Works for historical and future dates with full accuracy
Common use cases
- Calculate how many days until a project deadline
- Find the day of the week for a date in the past
- Convert meeting times across different time zones
- Calculate age from a date of birth
- Add or subtract business days for contract terms
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