Day of the Week Calculator: What Day Was Any Date?

Day of the Week Calculator

Select any date and instantly see which day of the week it falls on, along with useful context: ISO week number, day of year, calendar quarter, and whether the year is a leap year.

Find the next weekday

Use the finder below the main result to answer questions like "When is the next Monday after this date?", useful for scheduling, planning deadlines, or figuring out meeting windows.

Common uses

• Check what day of the week a birthday, holiday, or anniversary falls on
• Plan ahead for weekly recurring events
• Verify historical dates (e.g. what day did the moon landing occur?)
• Calculate ISO week numbers for business reporting

Zeller's Congruence

Zeller's Congruence is a mathematical algorithm to compute the day of the week for any date in the Gregorian calendar. The formula:

h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7

• h: day of the week (0 = Saturday, 1 = Sunday, ..., 6 = Friday)
• q: day of the month
• m: month (3 = March, 4 = April, ..., 14 = February). January and February are counted as months 13 and 14 of the previous year.
• K: year of the century (year mod 100)
• J: zero-based century (⌊year/100⌋)

Invented by Christian Zeller in 1882, this formula handles Gregorian calendar leap year rules and century adjustments.

Notable dates in history

Doomsday algorithm

The Doomsday algorithm, invented by John Conway, is a mental calculation method to find the day of the week for any date. The core idea: certain "Doomsdays" in each year always fall on the same day of the week:

• 4/4, 6/6, 8/8, 10/10, 12/12 (April 4, June 6, etc.)
• 5/9, 9/5, 7/11, 11/7 (May 9, Sept 5, July 11, Nov 7) - mnemonic: "I work 9-to-5 at the 7-Eleven"
• Last day of February (Feb 28 or 29)

To use the algorithm: memorize the Doomsday for the target year's century, adjust for the year within that century, then count days from the nearest known Doomsday to your target date. With practice, you can compute the day in under 5 seconds.