MasterTool

Day of the Week Calculator

Find what day of the week any date fell on — or will fall on — including your birthday, a historical date, or a future deadline.

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What this tool does

Find what day of the week any date fell on — or will fall on — including your birthday, a historical date, or a future deadline. Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.

How to use the Day of the Week Calculator

  1. Enter or select date (yyyy-mm-dd).
  2. Read the calculated result; change any measurement to compare alternatives.

Formula

weekday = UTC day-of-week of the entered Gregorian calendar date (the same value Zeller's congruence and known-date algorithms produce)
date
Date (YYYY-MM-DD)

Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.

Worked example

For day of the week calculator, the following measurements illustrate the exact method: Date (YYYY-MM-DD): 2026-01-01.

Inputs

  • Date (YYYY-MM-DD)2026-01-01

Result

  • Day of the weekThursday
  • Full dateThursday, January 1, 2026
  • ISO day number (Monday = 1)4
  • Day of year1
  • WeekendNo — a weekday

Results explained

Day of the week
Day of the week from the formula above. Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.
Full date
Full date from the formula above. Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.
ISO day number (Monday = 1)
ISO day number (Monday = 1) from the formula above. Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.
Day of year
Day of year from the formula above. Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.
Weekend
Weekend from the formula above. Uses the proleptic Gregorian calendar in UTC, so the weekday never shifts with your timezone. Dates before the Gregorian calendar's 1582 adoption are projected backwards using Gregorian rules.

Frequently asked questions

Enter your date of birth and the tool returns the weekday directly — for example 1990-06-15 was a Friday. No year-by-year counting is needed because the calculation is done on the calendar date itself.

Every calendar date maps to exactly one weekday in the repeating 7-day cycle. The tool reads the weekday of the entered date in UTC using Gregorian calendar rules — the same result Zeller's congruence gives by hand.

Yes. Enter any date — a wedding, a deadline, a historical anniversary — and the weekday is computed the same way for past and future dates.

The input is a date only, with no time, and the weekday is evaluated at UTC midnight. A date-only value can never slip into the neighbouring day the way a timestamp near midnight can.

They use the proleptic Gregorian calendar (Gregorian rules projected backwards). Countries adopted the Gregorian calendar at different times, so a date recorded under the Julian calendar may differ by a few days from its Gregorian projection.