Day of Week

Find the day of the week for any date, past or future. Enter a date to check what day your birthday falls on, verify historical dates or plan upcoming events with certainty.

Select a date to find the day of the week

How to Use

  1. Enter a date

    Input the year, month, and day of the date you want to check.

  2. Run the calculation

    Click Calculate to determine the day of the week.

  3. View the result

    The day of the week is displayed for the date you entered.

What is a day of the week?

A day of the week tells you which of the seven days — Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday — a given date falls on. The seven-day cycle dates back to the astral beliefs of ancient Babylon and carried over into the Gregorian calendar, repeating without interruption regardless of leap years or century adjustments.

This calculator shows the day of the week for the date you enter in both Korean and English and even tells you whether it is a weekend (Saturday or Sunday). It is handy for checking what day a birthday or anniversary fell on, working out in advance whether a future event or deadline lands on a weekday or weekend, or figuring out the day of a historical event. To avoid time-zone errors, the entered date is interpreted at local midnight.

How it is calculated

Computers determine the weekday by converting a date into a day index (0–6). This calculator uses JavaScript's Date.getDay().

day index = getDay(year-month-day) // 0=Sunday … 6=Saturday

For example, entering 2024-03-01 returns index 5, which is shown as Friday; since 5 is neither Saturday (6) nor Sunday (0), it is judged a weekday. Mathematically, Zeller's congruence h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7 gives the same result, where q is the day, m is the month (starting at March = 3), K is the last two digits of the year, and J is the first two digits.

Frequently Asked Questions

How do I calculate the day of the week for a specific date?
The easiest way is to enter the date into this calculator. Mathematically, you can use Zeller's congruence or the Doomsday algorithm; internally, a computer converts the date into a number and takes the remainder after dividing by 7 to determine the weekday.
What is Zeller's congruence?
It is a weekday formula devised by the 19th-century German mathematician Christian Zeller. Using only the year, month, and day, it derives the weekday through arithmetic, expressed as h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7. Here q is the day, m is the month (starting at March = 3), K is the last two digits of the year, and J is the first two digits.
How does the weekday change in a leap year?
Normally the weekday of the same date shifts back by one day each year. If 1 March is a Monday this year, it becomes a Tuesday next year. But in a leap year (one with 29 February), weekdays from March onward shift by two days. As a result, the cycle in which the exact same weekday pattern repeats, including leap years, is 28 years.
Can I check the weekday for past dates too?
Yes, it accurately calculates the weekday for any past or future date based on the Gregorian calendar. Note, however, that dates from before 1582, when the Julian calendar was in use, may differ from the calendar of the time, so they should be read as Gregorian-converted dates.
Why is 0 Sunday in the weekday index?
This calculator follows the convention of starting the week on Sunday, assigning Sunday as 0 and Saturday as 6. It is judged a weekend when the value is Saturday (6) or Sunday (0). Note that the index basis differs from the ISO-8601 system, which treats Monday as the start of the week.
Can I check the weekday for lunar calendar dates?
This calculator works on the solar (Gregorian) calendar. For a lunar date, first convert it to the solar calendar and then enter it to find the weekday accurately.
Does my birthday return to the same weekday every four years?
Not necessarily. In common years the weekday shifts by one slot, and after 29 February in a leap year it shifts by two, so the interval before the same weekday returns is usually 5–6 years and not regular. The full cycle in which the entire weekday pattern, including leap years, repeats completely is 28 years.
2026 calendar data

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