Day of Week Calculator
Find what day of the week any date falls on—past, present, or future. Perfect for planning events and historical curiosity.
Day of Week Examples
| Date | Day |
|---|---|
| July 4, 1776 | Thursday |
| July 20, 1969 | Sunday |
| Jan 1, 2000 | Saturday |
| Dec 25, 2024 | Wednesday |
Implementation
``javascript
function getDayOfWeek(year, month, day) {
const date = new Date(year, month - 1, day);
const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
return days[date.getDay()];
}
`
Interesting Day Facts
The Gregorian calendar repeats every 400 years- Any date falls on each day of the week 58 times per 400 years
- The 13th falls on Friday slightly more often than other days
Working Out the Day of the Week
The Gregorian calendar repeats exactly every 400 years — 146,097 days, which is divisible
by 7. That means every date's weekday is fully determined, and the calendar for 2026 is
identical to 1626.
Zeller's congruence computes it by hand:
`
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7
`
where q is the day, m the month (March = 3, and January and February count as months 13 and
14 of the *previous* year), K the year within the century, and J the century. The month
offset exists because the Roman calendar started in March, which is also why September,
October, November and December are named for seven, eight, nine and ten.
In code you would just use getDay() — 0 is Sunday — but the ISO 8601 convention counts
Monday as 1 through Sunday as 7, and mixing the two conventions is a common source of
off-by-one weekday bugs.
The Traps in Date Arithmetic
Months are not 30 days. Adding one month to 31 January has no correct answer, and every
library picks differently — 28 February, 2 March or an error. Decide which your business
needs before the edge case decides for you.
Days are not always 86,400 seconds. Daylight saving makes one day 23 hours and another
25. Adding n * 86400000 milliseconds drifts by an hour twice a year, which is enough to
move a date across midnight and return an answer one day out.
`javascript
// Correct — the Date object normalises overflow and handles DST
const addDays = (date, days) => {
const result = new Date(date);
result.setDate(result.getDate() + days);
return result;
};
// Correct — calendar days, unaffected by DST or time of day
const daysBetween = (a, b) => {
const utc = (d) => Date.UTC(d.getFullYear(), d.getMonth(), d.getDate());
return Math.round((utc(b) - utc(a)) / 86400000);
};
`
Months are zero-indexed in JavaScript. new Date(2026, 0, 1) is January. new
Date(2026, 12, 1) is January 2027, silently.
Parsing is not portable. new Date("2026-08-30") is parsed as UTC, while
new Date("2026/08/30")` is parsed as local — the same calendar date, a day apart in some
zones. Always parse ISO 8601, and construct with explicit components when the time matters.
Time Zones Decide the Answer
| Question | Depends on |
|---|---|
| How many days until an event? | The viewer's zone |
| Is this subscription expired? | UTC, always |
| What day of the week was this? | The zone the event happened in |
| When does "tomorrow" start? | The viewer's zone |
Reference Points
| Interval | Days |
|---|---|
| Week | 7 |
| Month (average Gregorian) | 30.44 |
| Quarter | 91.31 |
| Year | 365.2425 |
| Leap year | 366 |