How to use the day of the week calculator
- Choose any date from year 1 to 9999 – a birthday, an anniversary, a historical event or a future appointment.
- Press Calculate.
- Read the weekday, which occurrence it is in the month (“3rd Sunday of October”), the day of the year, the ISO week number and how far the date is from today.
Find the weekday by hand: Zeller's formula
Zeller's congruence works out the weekday of any Gregorian date with simple arithmetic:
\begin{aligned} h = \Big(&q + \left\lfloor \frac{13(m+1)}{5} \right\rfloor + K \\ &+ \left\lfloor \frac{K}{4} \right\rfloor + \left\lfloor \frac{J}{4} \right\rfloor + 5J\Big) \bmod 7 \end{aligned}q– day of the monthm– month, March = 3 … December = 12; January and February count as 13 and 14 of the previous yearK– last two digits of the year;J– first two digits\lfloor x \rfloor– round down;\bmod 7– remainder after dividing by 7- Result:
h= 0 Saturday, 1 Sunday, 2 Monday, 3 Tuesday, 4 Wednesday, 5 Thursday, 6 Friday
q = 20,\; m = 7,\; K = 69,\; J = 19
\begin{aligned} h = \Big(&20 + \left\lfloor \frac{13 \times 8}{5} \right\rfloor + 69 \\ &+ \left\lfloor \frac{69}{4} \right\rfloor + \left\lfloor \frac{19}{4} \right\rfloor + 5 \times 19\Big) \bmod 7 \end{aligned}\begin{aligned} h &= (20 + 20 + 69 + 17 + 4 + 95) \bmod 7 \\ &= 225 \bmod 7 = 1 \end{aligned}h = 1, so the date was a Sunday.
A mental shortcut: the Doomsday rule
In any year, these easy-to-remember dates all fall on the same weekday, called the year's “doomsday”:
- 4/4, 6/6, 8/8, 10/10, 12/12
- 5/9 and 9/5, 7/11 and 11/7
- The last day of February
- March 14 (Pi Day)
In 2026 the doomsday is Saturday. To find another date, count from the nearest doomsday. For example, December 12, 2026 is a Saturday, so December 25 is 13 days later.
Where the weekday names come from
| Day | Named after | Latin name |
|---|---|---|
| Sunday | The Sun | dies Solis |
| Monday | The Moon | dies Lunae |
| Tuesday | Tiw (Týr), Germanic god of war | dies Martis |
| Wednesday | Woden (Odin) | dies Mercurii |
| Thursday | Thor, god of thunder | dies Iovis |
| Friday | Frigg, Germanic goddess | dies Veneris |
| Saturday | Saturn, Roman god | dies Saturni |
Old dates and the calendar change
This calculator uses the Gregorian calendar for every date, extended backwards before it existed. Countries switched from the older Julian calendar at different times, skipping several days when they did:
| Year of change | Countries |
|---|---|
| 1582 | Italy, Spain, Portugal and Poland |
| 1752 | Great Britain and its colonies (September 2 was followed by September 14) |
| 1918 | Russia |
| 1923 | Greece |
The weekday sequence was never interrupted, but a date written in a country's records before its switch may not match the Gregorian date shown here.
Patterns worth knowing
- A date moves forward one weekday each year, or two when February 29 falls in between – so birthdays drift through the week.
- The whole calendar repeats exactly every 400 years.
- A month that starts on a Sunday always has a Friday the 13th.
Frequently asked questions
What day of the week was I born?
Enter your date of birth and press Calculate. The Age Calculator also shows it alongside your exact age.
What day of the week will my birthday be next year?
Usually one weekday later than this year, or two if February 29 falls between them. Enter the date above to check.
How far back does the calculator work?
Any date from year 1 to 9999, using the Gregorian calendar.
What does “3rd Sunday of October” mean?
It counts which occurrence of that weekday the date is within its month – useful for holidays set that way, such as “the second Sunday of May”.
What day of the year is it?
The result shows the day number, from 1 (January 1) to 365, or 366 in a leap year, and how many days are left in the year.