Day of Week Calculator
Find the day of the week for any date, plus ISO week and day-of-year.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this day of week calculator
- Pick the date you want to look up.
- Press "Find day of week".
- Read the weekday name, plus day-of-year and ISO week number.
- Use Reset to enter a new date or Copy to share the answer.
About this day of week calculator
This tool tells you which weekday falls on any Gregorian calendar date — past or future — using Zeller’s congruence, a classic 1800s formula by Christian Zeller. The algorithm shifts January and February into the previous year, then combines the day, month, year-of-century, and century into a single modular arithmetic expression that yields a number from 0 to 6. The tool maps that number to Saturday through Friday, and re-maps to the familiar Sunday-first weekday order for display.
Because the calculation is pure integer arithmetic with no calendar lookups, it is fast and exact for every Gregorian date you can express in YYYY-MM-DD. The day is interpreted as a plain calendar date with no timezone offset — handy if you want to know which weekday a holiday falls on without worrying about UTC vs local time.
Worked example: enter 2026-07-04. The tool reports Saturday, day-of-year 185, and ISO week 27. That means the 4th of July 2026 lands on a Saturday — useful for trip planning, scheduling work shifts, or settling a "what day was I born?" question.
The day-of-year and ISO week number are computed alongside so you can cross-check spreadsheets or planning tools that index by week.
FAQ
- Does this work for historical dates?
- Yes for Gregorian dates (1583 onwards in most countries). For earlier dates, the historical calendar in use was usually Julian, so the weekday this tool reports is the mathematically correct Gregorian projection rather than the day people actually observed.
- What is an ISO week?
- ISO 8601 weeks start on Monday and the first week of the year is the one containing the first Thursday. It is a stable numbering used in business, ERP, and scheduling tools.
- Is the date affected by my timezone?
- No. The date you pick is treated as a calendar date, so the same input always gives the same weekday regardless of where you are.
- Why Zeller’s congruence?
- It is exact, integer-only, and very fast. Modern Date libraries would also work, but Zeller is small enough to fit in a few lines and survives copy-paste into any language.