What Day Is It? Calculator

How to use it

1. Pick a date, or use Today / Tomorrow / Yesterday. The weekday appears at once.
2. Step a day back or forward with the arrows, or jump to the same date in another year from the list.
3. Read the extras: day of the year, days left in the year, ISO week, leap-year status and days since/until today.
4. Copy the one-line summary or the link to this date.

Example

Zeller's congruence

January and February are counted as months 13 and 14 of the previous year, so the leap day always sits at the end of the “year”. With q = day of the month, m = month (3–14), K = year of the century and J = zero-based century:

h = (q + ⌊13(m + 1) / 5⌋ + K + ⌊K / 4⌋ + ⌊J / 4⌋ + 5J) mod 7
0 = Saturday, 1 = Sunday, 2 = Monday, … 6 = Friday

For 20 July 1969: q = 20, m = 7, K = 69, J = 19 → 20 + 20 + 69 + 17 + 4 + 95 = 225; 225 mod 7 = 1 → Sunday. The term ⌊13(m + 1)/5⌋ reproduces the 31/30-day pattern of the months, ⌊K/4⌋ and ⌊J/4⌋ − 2J (rewritten as +5J mod 7) add the leap days of the Gregorian rule.

Gauss’s algorithm

Gauss’s method gives the weekday of 1 January: (1 + 5·((Y−1) mod 4) + 4·((Y−1) mod 100) + 6·((Y−1) mod 400)) mod 7 with 0 = Sunday; the day of the year is then added. Both formulas give identical results, and the implementation is verified against the JavaScript Date object for every day of several years including century boundaries (1600, 1700, 1900, 2000, 2100, 2400).

The calendar is the proleptic Gregorian one: dates before the Gregorian reform (Britain switched in 1752, Russia in 1918) are shown as they would be in the Gregorian calendar, not as they were written at the time, which is why the tool starts at 1583. ISO weeks start on Monday; week 1 contains the first Thursday of the year.

Sources: Zeller’s congruence, Determination of the day of the week (incl. Gauss’s algorithm), ISO 8601 week dates.

Questions

What day of the week was I born on?

Enter your date of birth: the big result is the weekday. The “same date in other years” list shows what day your birthday falls on this year and next.

Why does the weekday move by one day each year, and by two after a leap year?

A common year has 365 days = 52 weeks + 1 day, so the same date lands one weekday later next year. A leap year has 366 days = 52 weeks + 2 days, so dates after 29 February jump two weekdays.

Can I check a date before 1583?

Not here. Before the Gregorian reform of October 1582 the Julian calendar was in use, with a different leap-year rule, and countries adopted the new calendar at different times, so a single weekday answer would be misleading.

Is the calculation done on a server?

No. The weekday is computed in your browser with Zeller’s congruence; the link only carries the date in its fragment.