How to use the Day of the Week Calculator
- Pick a date with the date field.
- Read the weekday in the first result.
- Check the day of the year and the ISO week number if you need them for planning or reports.
- Change the date to look up as many as you like.
Formula used
The weekday comes from the calendar (Gregorian) rules. ISO week 1 is the week with the first Thursday of the year; weeks start on Monday.
Worked example
Here is the Day of the Week Calculator worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| Date | 2027-01-01 |
| Result | |
| Day of the week | Friday |
| Day of the year | 1 |
| ISO week number | 53 |
| Days left in the year | 364 |
| Leap year? | No |
What you get
For 1 January 2027, the weekday is Friday. It is day 1 of the year, and it falls in ISO week 53, because the first days of January can belong to the last week of the previous year. There are 364 days left in the year, and 2027 is not a leap year.
Some other dates: 29 February 2000 was a Tuesday, 31 December 2024 was day 366 of a leap year, and 3 January 2021 fell in ISO week 53.
How the weekday is found
The calendar repeats every 400 years in the Gregorian system, which is the calendar used almost worldwide. The calculator uses the standard calendar rules to count days from a known reference date and finds the remainder when divided by seven.
It applies the Gregorian calendar to every date, including dates before it was adopted in some countries. For historical dates before the local switch, the weekday this gives may differ from the one written in old records.
Leap years
A year is a leap year if it is divisible by 4, except that years divisible by 100 are not leap years unless they are also divisible by 400. So 2024 and 2028 are leap years, 1900 was not, and 2000 was. A leap year has 366 days, with 29 days in February.
That is why the number of days left in the year depends on the year: 365 days remain after 1 January 2024, but 364 days remain after 1 January 2025.
ISO week numbers
The ISO 8601 standard, used in business planning, numbers weeks from 1, with weeks starting on Monday. Week 1 is the week that contains the first Thursday of the year. This means some years have 53 weeks, and the first days of January can fall in the last week of the year before, as with 1 January 2027 in week 53 of 2026.
Some countries and calendar programs number weeks differently, for example starting weeks on Sunday, so check which system your organisation uses.
Frequently asked questions
How does it work for old dates?
It uses the Gregorian calendar for every date, including dates before it was adopted in some countries.
What is the ISO week number?
The international standard numbering of weeks, where weeks start on Monday and week 1 contains the first Thursday of the year.
How do I know if a year is a leap year?
A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400.
Why can week 53 exist?
Some years have 53 ISO weeks, because 52 weeks are 364 days and a year has 365 or 366.
Can I find the weekday of my birthday?
Yes. Enter your date of birth and read the weekday.
Does it work for future dates?
Yes, for any valid date in the calendar.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.