Weekday Calculator
Weekday Calculator
Find Any Day of the Week
Quickly determine what day of the week any date falls on. Perfect for planning events, checking birthdates, or historical research.
Also shows the week number, day of year, and whether the date falls on a weekend.
What the Weekday Calculator Tells You
Pick any calendar date and the tool returns five facts at once: the day of the week, the day-of-week number (0 for Sunday through 6 for Saturday, the convention used in most programming languages), the week of the year, the day of the year (1–365, or 1–366 in leap years), and how many days remain until December 31. A weekend flag highlights Saturdays and Sundays in green so they stand out immediately.
How Weekday Calculation Works
The Gregorian calendar is perfectly periodic: 400 years contain exactly 146,097 days, which is exactly 20,871 weeks. That means the entire weekday pattern repeats every 400 years — March 15, 2025 falls on the same weekday as March 15, 2425. Within shorter spans, a normal year shifts every date forward by one weekday (365 = 52 weeks + 1 day) and a leap year shifts dates after February by two. Mental-math methods like John Conway’s “Doomsday rule” exploit these patterns; this calculator simply computes the result directly from the date, which is faster and error-free.
Worked Examples
- July 20, 1969 (Apollo 11 moon landing) — a Sunday.
- January 1, 2000 (the millennium) — a Saturday.
- December 25, 2030 — a Wednesday, useful to know years before booking holiday travel.
Understanding Day of Year and Week Number
The day of year is handy for scientific and logistics work — many data formats and satellite products index dates as “day 032” rather than February 1. The week number shown here counts weeks from January 1 with Sunday as the first day of the week. Note that this differs slightly from the ISO 8601 standard used in much of Europe, where weeks start on Monday and week 1 is the week containing the year’s first Thursday, so the two systems can disagree by one week near the start and end of a year.
Practical Uses
- Event planning: check whether a future wedding date, product launch, or anniversary lands on a weekend.
- Historical research and genealogy: find the weekday of a birth, marriage, or historical event from old records.
- Contracts and deadlines: confirm whether a due date falls on a business day.
- Payroll and scheduling: use the week number and days-remaining figures for period-based planning.
Frequently Asked Questions
How do I find out what day of the week a date falls on?+
Select any date in the date picker (or click Today) and the calculator instantly shows the weekday in large text, color-coded green if it is a weekend and blue if it is a workday. It works for past dates like birthdays and historical events as well as future dates for planning.
Why does the same date fall on a different weekday each year?+
A common year has 365 days, which is 52 weeks plus one extra day, so every date moves forward one weekday from year to year. After a leap day the shift is two weekdays. The full Gregorian pattern repeats exactly every 400 years, since 400 years contain a whole number of weeks (20,871).
What does the week number mean, and is it the same as ISO week numbers?+
The week number counts weeks from January 1 with Sunday as the first day of the week. The ISO 8601 system used in many European countries instead starts weeks on Monday and defines week 1 as the week containing the first Thursday of the year, so results can differ by one week near year boundaries.
What is the day-of-year value useful for?+
Day of year (sometimes called the ordinal date or Julian day of year) numbers each day from 1 to 365 or 366. It is widely used in science, aviation, and logistics — for example February 1 is day 32 — and the companion days-remaining figure tells you how much of the year is left.
Is the weekday calculator free and does it work on my phone?+
Yes. It is completely free, needs no account, and all calculations run locally in your browser — no date you enter is sent anywhere. The layout is responsive, so it works just as well on a phone or tablet as on a desktop.
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