Day of the Week Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
The day of the week for a date follows the Gregorian calendar. January 1, 2026 falls on a Thursday, in ISO week 1, and it's the 1st day of the year.
Explanation
This calculator gives three complementary readings of a date: the day of the week (Monday through Sunday), its week number under the ISO 8601 standard, and its day-of-year count — its position in the year, from 1 (January 1) to 365 or 366 (December 31). The ISO 8601 week number follows a precise convention that differs from what many people assume: weeks start on Monday (not Sunday), and week 1 of each year is the one containing the first Thursday of January — which is why the very first days of January sometimes still belong to the last week of the previous year, and why some years have 53 ISO weeks instead of 52 (like 2026, which happens to start on a Thursday). This convention is widely used in business, project planning, and European professional calendars, unlike the 'American' week number (weeks starting on Sunday), which gives different numbers for the same dates. To calculate a number of days between two dates rather than the position of a single date, see our date difference calculator.
Example: March 1, 2026
Inputs
Date: March 1, 2026.
Calculation
March 1, 2026 falls on a Sunday. It's the 60th day of the year 2026 (31 days of January + 28 days of February, 2026 not being a leap year, + 1), and it belongs to ISO week 9.
Result
March 1, 2026 is a Sunday, the 60th day of the year, in ISO week 9.
Frequently asked questions
Why doesn't the ISO week number always match what I expect?
The ISO 8601 standard makes weeks start on Monday and defines week 1 as the one containing the first Thursday of the year (equivalently, the one containing January 4). Concretely, January 1st, 2nd, or 3rd can sometimes belong to the last week of the previous year rather than week 1 of the new year, depending on which day of the week January 1st falls on.
Why do some years have 53 ISO weeks instead of 52?
A year has 53 ISO weeks when it starts on a Thursday (like 2026), or when it's a leap year that starts on a Wednesday. In other cases, the year has exactly 52 complete ISO weeks.
What's the difference with the week number shown by my phone or Excel?
Some systems (notably in the United States) use a different convention where weeks start on Sunday and week 1 is simply the one containing January 1st, with no reference to Thursday. This convention can give a week number offset by 1 compared to the ISO 8601 standard used here, which is the reference in Europe.