Day of the Week Calculator
Enter any date to instantly find which day of the week it falls on. The calculator shows the full day name, how many days into the year that date is, which occurrence of that weekday it is within the month, how many times that weekday appears in the month, and a fact about the day. It works for any Gregorian calendar date, including dates centuries in the past or far into the future.
How to find the day of the week for any date
Select a year, month, and day, and the calculator returns the weekday instantly using Tomohiko Sakamoto's algorithm, a compact closed-form formula that works for any date in the Gregorian calendar from 1 AD onward. Beyond the day name, you get the day's ordinal position within the year, which occurrence of that weekday it is in the month (for example, the 3rd Wednesday), how many times that weekday appears in the month, and whether the year is a leap year. A weekday fact rounds out each result.
The algorithm: how the day is calculated
Sakamoto's algorithm maps a Gregorian date to a weekday index (0 = Sunday through 6 = Saturday) using a single arithmetic expression. It adjusts the year for January and February, applies a precomputed month-offset table that absorbs the irregular month lengths, then sums the adjusted year, its century-leap adjustments (divide by 4, subtract divide by 100, add divide by 400), the month offset, and the day, finally taking the remainder when divided by 7. The result directly encodes the weekday without requiring any lookup tables beyond the seven month offsets.
Julian vs. Gregorian calendar
This calculator uses the proleptic Gregorian calendar, meaning it extends the Gregorian rules backward to dates before the calendar was adopted in 1582. Historically, much of the world used the Julian calendar before that point, which places dates roughly 10 to 13 days earlier. If you are researching a historical event from before the calendar reform, the weekday shown here may differ from contemporary historical records, which were written in the Julian system.
Where weekday names come from
The seven-day week and the names of its days come from the ancient world. Romans named the days after the seven celestial bodies visible to the naked eye: the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. When Germanic peoples adopted the seven-day week, they replaced some Roman god names with their Norse equivalents: Mars became Tyr (Tuesday), Mercury became Woden/Odin (Wednesday), Jupiter became Thor (Thursday), and Venus became Frigg or Freya (Friday). Saturday kept its Latin form (Saturn's day), and Sunday and Monday kept their celestial names across most languages.
Days of the Week: Names, Origins, and Type
| Day | Named After | Latin Root | Type |
|---|---|---|---|
| Sunday | The Sun | dies Solis | Weekend |
| Monday | The Moon | dies Lunae | Weekday |
| Tuesday | Tyr (Norse) | dies Martis | Weekday |
| Wednesday | Woden / Odin | dies Mercurii | Weekday |
| Thursday | Thor (Norse) | dies Iovis | Weekday |
| Friday | Frigg / Freya | dies Veneris | Weekday |
| Saturday | Saturn | dies Saturni | Weekend |
Each weekday name traces back to a celestial body or deity in Roman or Norse mythology.
Frequently asked questions
What day of the week was I born?
Enter your birth year, month, and day into the calculator and it will tell you instantly. The algorithm works for any Gregorian date from 1 AD through 9999, so any birth date in recorded history is supported.
Does this calculator work for dates before 1582?
Yes, but with an important caveat. The calculator uses the proleptic Gregorian calendar, extending modern leap-year rules backward before the Gregorian calendar was actually adopted. Historical dates before 1582 in Europe were recorded in the Julian calendar, which has different leap-year rules and is now about 13 days behind. If you are checking a medieval or ancient date, the weekday may not match what contemporaries would have experienced.
What is the difference between a weekday and a weekend?
In the standard international (ISO 8601) convention, Monday through Friday are weekdays and Saturday and Sunday form the weekend. Some cultures and religions define their rest day differently: in Israel the Sabbath runs from Friday evening to Saturday night, and in many Islamic countries the weekend falls on Friday and Saturday.
How do I find the 3rd Thursday of a month?
Enter the year and month, then try days 15 through 21. The result will show the occurrence of each Thursday (1st, 2nd, 3rd, or 4th). Alternatively, find the first Thursday (days 1-7) and add 14 days to land on the 3rd occurrence.
Why does February sometimes have 29 days?
A solar year is approximately 365.2425 days long. To keep the calendar aligned with the seasons, an extra day is added to February every four years (a leap year). The rule is: divisible by 4 is a leap year, except years divisible by 100 are not, except years also divisible by 400 are. So 2000 was a leap year, 1900 was not, and 2100 will not be.
Is Sunday the first or last day of the week?
It depends on the convention. The international standard ISO 8601 defines Monday as the first day and Sunday as the seventh. However, in the United States, Canada, and many other countries, weekly calendars traditionally start on Sunday. Most religious calendars (Jewish, Christian, Islamic) also treat Sunday as the first day of the week.