Day of the Week Calculator
Enter a date to find out which day of the week it falls on.
Your calculation is performed in your browser.How it works
The calculator normalizes your chosen date to a calendar day and reads off its weekday directly from the calendar, so it works identically for dates far in the past or future.
Formula & methodology
The weekday repeats every 7 days, so it can be found by counting days from a date whose weekday is known. The calculator does this internally and reports both the weekday name and the ISO weekday number (Monday = 1 through Sunday = 7).
Weekday index
Weekday = (4 + Days since 1 Jan 1970) mod 7ISO weekday
ISO number = Weekday, but Sunday (0) is shown as 7What each term means
- The number 4
- 1 January 1970 was a Thursday, which is index 4.
- Days since 1 Jan 1970
- Whole days from that reference date to your date.
Weekday index
| Index | Day |
|---|---|
| 0 | Sunday |
| 1 | Monday |
| 2 | Tuesday |
| 3 | Wednesday |
| 4 | Thursday |
| 5 | Friday |
| 6 | Saturday |
Worked example
July 20, 1969 was a Sunday.
Common uses
- —Finding out what day of the week you (or someone else) were born on.
- —Checking whether a historical or future date falls on a weekday or weekend.
- —Verifying the weekday for scheduling, legal documents, or record-keeping.
Important notes
- —Works for any date the browser can represent, including dates well before or after today.
- —The result also flags whether the date falls on a weekend (Saturday or Sunday).