Day of Week Calculator
Find the weekday, ISO week number, and day of year for any date — or look up the next or previous occurrence of any weekday.
Day of week
Monday
- ISO week number
- Week 25 of 2026 (ISO 8601)
- Day of year
- Day 166 of 2026
How to Use
Mode 1 — Date to Weekday: Type or pick any date in the date field and instantly see its day of the week, ISO week number, and how many days into the year it falls. Today's date is pre-filled so you can start right away. Great for checking the weekday of a birthday, anniversary, historical event, or any upcoming deadline.
Mode 2 — Weekday to Date: Select a weekday and choose 'Next' or 'Previous' to find the closest future or past occurrence of that day, calculated from today. If today already falls on the selected weekday, the tool shows 'Today is [day]'. No need to flip through a calendar.
Common use cases: confirming what day a 'next Monday' meeting falls on, looking up what day of the week you were born, verifying the ISO week number shown in a spreadsheet or project management tool, or quickly finding the nearest Friday for a pay date or due date.
How It Works
ISO Week Number — The Year-Boundary Quirk: The ISO 8601 standard defines the first week of the year as the week containing January 4th, and all weeks start on Monday. This seemingly simple rule has a counterintuitive consequence: dates in late December can belong to the following year's week 1, and dates in early January can belong to the previous year's last week. For example, December 29, 2025 (Monday) is classified as ISO 2026 week 1. Some years even have 53 weeks instead of 52 — 2026 is one such year, because December 31 falls on a Thursday. This '53-week year' is a well-known source of bugs in scheduling software and fiscal-year systems. This tool handles all edge cases in strict compliance with ISO 8601.
Zeller's Congruence vs. Modern JavaScript: Before computers, the day of the week for any Gregorian date was calculated using Zeller's congruence, a formula devised by 19th-century mathematician Christian Zeller: h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7 (where q = day, m = month with Jan/Feb counted as months 13/14 of the prior year, K = last 2 digits of year, J = century). Today the same result can be obtained in a single line of JavaScript using Date.UTC() and getUTCDay(). This tool calculates everything in UTC to be completely immune to time-zone and daylight-saving-time shifts — you'll get the same answer no matter where in the world you open it.
FAQ
- What is an ISO week number, and how does it differ from a regular week number?
- The ISO week number (defined in ISO 8601) always starts on Monday, and the first week of the year is the one containing January 4th. Because of this definition, late-December dates can belong to the following year's week 1 and early-January dates can belong to the previous year's last week — for example, January 1, 2022 (Saturday) is ISO 2021 week 52. In Excel or Google Sheets, use =WEEKNUM(date, 21) to get an ISO-compliant week number; the default WEEKNUM formula uses a Sunday-based, non-ISO system.
- How do I find the next or previous occurrence of a weekday?
- Switch to the 'Weekday → Date' tab. Select the weekday you want and choose 'Next' or 'Previous'. The tool calculates the closest future or past date for that weekday from today. If today is already that weekday, it shows 'Today is [day]' instead. The calculation automatically crosses month and year boundaries as needed.
- Does January 1 always fall in week 1?
- Not in ISO 8601. January 1 falls in the previous year's last week whenever it lands on Friday, Saturday, or Sunday — because the week containing it doesn't include January 4th. For instance, January 1, 2022 (Saturday) belongs to ISO 2021 week 52. If your calendar app or planner shows a different week number than this tool, check whether the other system uses ISO 8601 (Monday start, Jan 4 rule) or a different convention.
- What's the difference between this tool and the date calculator?
- Use this tool when you want to know what day of the week a specific date falls on, or when you need to find the nearest Monday (or any other weekday). Use the date calculator when you need to count how many days are between two dates or calculate a date that is N days from a starting point.