Day of Week Calculator

Knowing the weekday of a date is essential for scheduling, historical research, and understanding patterns like 'first Monday of the month'. This calculator returns the weekday name, the ordinal day of the year (1–366), and the ISO week number (1–53) for any date you enter.

Enter values

Results

Day of the week: Thursday
Day of the year: 228
Week number (ISO): 33

How is the Day of Week calculated?

Enter the day, month and year. The calculator builds the date in UTC, then reads the weekday (0 = Sunday … 6 = Saturday), counts days from 1 January for the day of year, and applies the ISO-8601 week definition (week 1 is the week containing the first Thursday). All results are based on the Gregorian calendar.

Formula

Weekday = UTC getUTCDay() (0 = Sunday). Day of year = days between date and 1 January of the same year + 1. ISO week number = week containing the first Thursday of the year; calculated by finding the Thursday of the target week and counting weeks from the first Thursday.

Example

15 August 2024: weekday = Thursday, day of year = 228 (leap year), ISO week = 33. 1 January 2024: weekday = Monday, day of year = 1, ISO week = 1. 29 February 2024: weekday = Thursday, day of year = 60, ISO week = 9. 31 December 2024: weekday = Tuesday, day of year = 366, ISO week = 1 — the last day of 2024 already sits in the first ISO week of 2025. 31 December 2023: weekday = Sunday, day of year = 365, ISO week = 52.

Things to remember

The ISO week number is the standard for business reporting and payroll in many countries. Day of year helps with 'day 100' milestones or fiscal calendars. For recurring events like 'third Friday', combine this with the add-days calculator.

Frequently Asked Questions

What exactly is the ISO week number?

An international standard where week 1 is the week containing the first Thursday of the year. Weeks start on Monday. This means 1–3 January can belong to week 52 or 53 of the previous year, and 29–31 December can belong to week 1 of the next year.

Why does the calculator use UTC?

To give a single, consistent answer regardless of your time zone. The weekday of a date is the same worldwide at UTC midnight.

Can I find the weekday for a future date?

Yes — any date from 1900 to 2100 is supported, past or future. The Gregorian calendar rules apply throughout, so leap days and century rules (2000 was a leap year, 1900 was not) are handled for you automatically.

Which calendar does this calculator use?

The Gregorian calendar, which is the civil calendar used in almost all countries today. Dates before 1582 (when it was introduced) are extended proleptically.