Weekday Calculator — Free Online
Find out what day of the week any date falls on — past or future. Instantly see the weekday, week number, day of year, and calendar quarter for any date.
How to Use the Weekday Calculator
- Enter any date using the date picker — works for any date from year 1 to year 9999
- Click "Find Weekday" to instantly see the result
- The result shows the full weekday name, ISO weekday number, week number, day of year, and quarter
- Weekend dates are flagged automatically so you can spot Saturdays and Sundays at a glance
- Use the copy button to copy the weekday name for use in documents
How the Weekday Calculator Works — The Formula
Tomohiko Sakamoto / Zeller variant:
d = (Y + ⌊Y/4⌋ − ⌊Y/100⌋ + ⌊Y/400⌋ + t[m] + day) mod 7
Where t[] = {0,3,2,5,0,3,5,1,4,6,2,4} for months Jan–Dec
And Y is decremented by 1 if month < 3
Result: 0=Sunday, 1=Monday, ..., 6=Saturday
Formula validated by Vipul Jaganiya, Computer Science expert in Calendar Mathematics and Timezone Engineering. Valid for all proleptic Gregorian dates.
Real-World Examples
| Date | Weekday | Week No. | Day of Year |
|---|---|---|---|
| 2026-01-01 | Thursday | Week 1 | Day 1 |
| 2026-02-14 | Saturday | Week 7 | Day 45 |
| 2026-08-15 | Saturday | Week 33 | Day 227 |
| 2026-12-25 | Friday | Week 52 | Day 359 |