# Day of the Week Calculator

URL: https://speedycalculator.com/calculators/everyday-life/day-of-week/
Last updated: 2026-10-25

> The day of the week calculator tells you which weekday any date falls on – past or future – along with its day of the year, ISO week number, which occurrence of that weekday it is in the month (e.g. 3rd Tuesday), and whether the year is a leap year.

## Inputs

- Date

## Results

- Day of the week
- Occurrence in the month
- Day of the year
- ISO week number
- Days in this month
- Quarter
- Leap year?

## Formula

```
Weekday from the Gregorian calendar (Zeller-type rule)
Occurrence in month = ceil(day of month ÷ 7)
ISO week = week containing the date's Thursday
```

## Explanation

Knowing the weekday of a date is handy for planning events, checking historical dates or working out "the third Monday of the month". The calculator uses the proleptic Gregorian calendar, which extends today's calendar rules backwards.

### How to use the Day of the Week Calculator

- **Date**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Pick any date.

### Tips

- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so "one month" is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.

### Common mistakes to avoid

- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).

### What each result means

- **Day of the week** = weekday([Date])
- **Occurrence in the month** = ceil(day([Date])÷7)
- **Day of the year** = dayofyear([Date])
- **ISO week number** = (floor((dayofyear(([Date]) - mod(weekday([Date]) + 6, 7) + 3) - 1)÷7) + 1)
- **Days in this month** = date(year([Date]), month([Date]) + 1, 1) - date(year([Date]), month([Date]), 1)
- **Quarter** = ceil(month([Date])÷3)
- **Leap year?** = isleap(year([Date]))

## Worked example

Inputs — Date: 2026-10-25.

Results — **Day of the week = Sunday**; Occurrence in the month = 4th; Day of the year = 298; ISO week number = 43; Days in this month = 31; Quarter = Q4; Leap year? = No.

## FAQ

**What day of the week was I born?**
Enter your date of birth – the weekday appears instantly.

**How do I find the 3rd Monday of a month?**
It is the Monday falling on day 15–21 of the month.

**Why do weekdays repeat every 28 years?**
Within a century, the calendar cycle of weekdays and leap years repeats every 28 years.

**What formula does the Day of the Week Calculator use?**
Weekday from the Gregorian calendar (Zeller-type rule).