📆 Day of the Week Calculator

What Day of the Week Is Any Date?

Finding the day of the week for any given date is one of the most common calendar questions people ask. Whether you’re planning a wedding and need to know what day your anniversary will fall on in ten years, checking what day of the week a historical event occurred, verifying a due date, or figuring out the day for a recurring appointment, this calculator handles it instantly for any date in the Gregorian calendar.

The calculation uses JavaScript’s built-in Date object, which internally implements the proleptic Gregorian calendar extended backward before its 1582 adoption. For dates from 1582 onward the results match the historical calendar. The algorithm is equivalent to Zeller’s Congruence, a mathematical formula developed by Christian Zeller in 1882 that computes the day of the week from the year, month, and day values using modular arithmetic.

The Seven Days: Origins and Character

Saturday is the only English day name derived directly from Latin rather than Germanic mythology. All other days reflect the Old English translation of the Roman planetary week, a system that mapped each day to a celestial body and its associated deity.

How the Day of the Week Is Calculated

Zeller’s Congruence

Christian Zeller published his congruence formula in 1882. It works by converting a date into a single integer modulo 7. For the Gregorian calendar, the formula adjusts for January and February being treated as months 13 and 14 of the previous year (to simplify the irregular leap-year handling), then applies century corrections that account for the Gregorian calendar’s century-year rule (century years are only leap years if divisible by 400).

The Doomsday algorithm

The Doomsday algorithm, created by mathematician John Conway, lets people calculate the day of the week mentally. It’s based on the observation that certain “anchor dates” in every year always share the same weekday, called the Doomsday. For example, 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February always fall on the same weekday each year. Once you know that year’s Doomsday, you can quickly find any date by counting from the nearest anchor.

Notable Days: What Weekday Are Major Holidays?

Calendar Curiosities

The Gregorian calendar repeats itself exactly every 400 years, meaning 2025 and 2425 have identical calendars. Within a single year, some weekdays appear 53 times and the rest appear 52. Which day gets the extra occurrence depends on what day January 1 falls on and whether it’s a leap year. In 2026, which starts on a Thursday, both Thursday and Friday appear 53 times.

Friday the 13th occurs at least once and at most three times every year. The maximum gap between two Friday the 13ths is 14 months. There’s never a year without at least one. The longest streak of months with Friday the 13th is three consecutive months, this happens when February has a Friday the 13th in a non-leap year starting on Thursday.