Day of the Week Calculator – What Day Was or Will It Be?

How to use the day of the week calculator

Pick a date — it starts on today. You’ll see:

• the day of the week,
• the day of the year,
• the ISO week number,
• whether it is a leap year,
• and the weekday of the same date over the next few and previous few years.

How the weekday is calculated

The calculator uses the Gregorian calendar, the one used almost everywhere today. One classic formula is Zeller’s congruence:

where q is the day of the month, m the month (March = 3 … February = 14, counted as part of the previous year), K the year of the century and J the century. The result h gives the weekday, with 0 = Saturday.

Worked example: 8 November 2026

q = 8, m = 11, K = 26, J = 20:

• ⌊13 × 12 ÷ 5⌋ = 31
• ⌊26 ÷ 4⌋ = 6 and ⌊20 ÷ 4⌋ = 5
• h = (8 + 31 + 26 + 6 + 5 + 100) mod 7 = 176 mod 7 = 1 → Sunday

Fun facts about weekdays

• The calendar repeats exactly every 400 years.
• Non-leap years starting on the same weekday share the same calendar: 2026 has the same calendar as 2015 and 2037.
• Friday the 13th happens at least once and at most three times a year. In 2026 it falls in February, March and November.
• The 13th of a month is very slightly more likely to be a Friday than any other weekday, over a 400-year cycle.

Leap year rules

The Gregorian calendar has a leap year every four years, except century years that don’t divide by 400. So 1900 was not a leap year, 2000 was, and 2100 won’t be. This keeps the calendar in step with the seasons to within about one day every 3,000 years, and it is why the weekday pattern only repeats exactly every 400 years.

The calculator applies these rules automatically, so every date you enter gets the right weekday.

Everyday uses

• Birthdays and anniversaries: what day of the week does your birthday fall on this year?
• Planning events: check that a wedding or party date lands on a weekend.
• History: discover the weekday of an important event.
• Deadlines: check whether a due date falls on a weekend.
• Puzzles and trivia.

To count the days between two dates, try the date difference calculator. For working days only, use the business days calculator, and for exact ages the age calculator.