Day of Week Calculator

It uses Zeller's congruence, a closed-form formula published by Christian Zeller in 1882. The formula combines the day, month, year, and century into a single arithmetic expression and returns a number from 0 to 6 for the weekday. The calculator runs Zeller's congruence directly instead of relying on the JavaScript Date object, so the result is independent of your device's time zone.

ISO 8601 (used across Europe and most of the world) says week 1 is the week that contains the first Thursday of the year, and weeks always start on Monday. US-style numbering treats week 1 as the week containing January 1, with weeks starting on Sunday. The two systems can disagree by one number near January, so a date that is ISO week 1 of 2025 may be US week 53 of 2024. This calculator reports the ISO 8601 week number.

A year is a leap year if it is divisible by 4, except for years divisible by 100, unless those years are also divisible by 400. So 2024 is a leap year (divisible by 4), 2100 is not (divisible by 100 but not 400), and 2000 is a leap year (divisible by 400). The rule keeps the Gregorian calendar in sync with the solar year over long periods.

Because the 100-year exception applies. The Gregorian rule drops three leap days every 400 years to correct for the fact that a solar year is about 365.2425 days, not exactly 365.25. Century years like 1700, 1800, and 1900 fail the divisible-by-400 test and skip their leap day. 2000 passes the test, so it kept its February 29.

Common uses include finding the weekday you were born on, checking what day a future birthday or anniversary falls on, planning project deadlines (e.g., confirming that a delivery date is a workday), scheduling around weekends and ISO calendar weeks, and answering trivia questions about historical events. It also handles dates centuries in the past or future, where calendar apps often stop.