About the Day of the Week Calculator
This day of the week calculator tells you what day any past or future date falls on: a birthday, a wedding date, a deadline or a historical event. It also shows the ISO week number (2026-W40 format, used in Europe and in many business tools), the US week number (weeks starting Sunday), the day of the year, the quarter and whether the date is, say, the 2nd Tuesday of the month. The table at the bottom shows the same date in other years, so you can see whether next yearโs birthday falls on a weekend.
How to use
- Type a date or pick one from the date picker. Todayโs date is shown first.
- Use the โน and โบ buttons to move one day back or forward.
- Read the day of the week, week numbers, day of the year and the month calendar.
- Check the table at the bottom for the same date in other years.
ISO week numbers vs. US week numbers
ISO 8601 week numbers start on Monday, and week 1 is the week that contains the yearโs first Thursday. If January 1 is a Friday, Saturday or Sunday, those days still belong to the last week (52 or 53) of the previous year. 2026 starts on a Thursday, so it has 53 ISO weeks.
The US convention (used by many American calendars and Excelโs default WEEKNUM) starts weeks on Sunday and always counts the week containing January 1 as week 1. The two systems often differ by one week, so check which one your company or school uses.
| Date | ISO week | US week |
|---|---|---|
| Thu, Jan 1, 2026 | 2026-W01 | Week 1 |
| Sun, Jan 4, 2026 | 2026-W01 | Week 2 |
| Thu, Oct 1, 2026 | 2026-W40 | Week 40 |
| Fri, Jan 1, 2027 | 2026-W53 | Week 1 |
How to work out the day of the week by hand (Zellerโs congruence)
For a Gregorian date, treat January and February as months 13 and 14 of the previous year, then compute h = (q + โ13(m+1)/5โ + K + โK/4โ + โJ/4โ + 5J) mod 7, where q is the day of the month, m the month, K the last two digits of the year and J the first two. h = 0 is Saturday, 1 Sunday, 2 Monday โฆ 6 Friday. This calculator uses your browserโs calendar and gives the same result instantly.
FAQ
Which years can I look up?
Any date from year 1000 to 9999. Dates before October 1582 use the current Gregorian calendar projected backwards, so they can differ from historical records kept in the Julian calendar (and Britain and its colonies only switched in 1752).
How many weeks are in a year?
Most years have 52 ISO weeks. Years that start on a Thursday, and leap years that start on a Wednesday, have 53. 2026 has 53 ISO weeks.
Why does the same date move to a different weekday every year?
A common year is 52 weeks plus 1 day, so a date moves one weekday later each year, or two days after a February 29. The same date-and-weekday combination usually comes back after 5, 6 or 11 years.