CalcLibrary

Day of Week Calculator

Enter one date and see which weekday it falls on. Useful for birthdays, anniversaries, deadlines, travel dates, and historical lookups.

Examples

Check which weekday a birthday landed on.

Weekday
Thursday
Day of year
307
Year type
Common year

Gregorian calendar model only. This calculator applies Gregorian rules consistently and does not model country-by-country historical adoption dates.

Was this useful?

Examples

How It Works

Formula

w=(y′+⌊y′4⌋−⌊y′100⌋+⌊y′400⌋+tm+d) mod 7w = \left(y' + \left\lfloor \frac{y'}{4} \right\rfloor - \left\lfloor \frac{y'}{100} \right\rfloor + \left\lfloor \frac{y'}{400} \right\rfloor + t_m + d\right) \bmod 7

leapYear=(y mod 400=0)∨((y mod 4=0)∧(y mod 100≠0))\text{leapYear} = (y \bmod 400 = 0) \lor ((y \bmod 4 = 0) \land (y \bmod 100 \ne 0))

Variables

ww

Weekday index, where 0 = Sunday and 6 = Saturday

y′y'

Adjusted year; January and February use the previous year

tmt_m

Month offset from the Gregorian weekday table

dd

Day of the month

yy

Gregorian calendar year of the entered date

Enter one date and the calculator resolves its weekday under a consistent Gregorian calendar model. The headline answer is the weekday label for that date, while the supporting rows keep just enough context to help with planning and fact-checking.

  1. Validate the entered date as one exact Gregorian calendar date.
  2. Shift January and February into the previous year for the weekday formula, then compute the weekday index.
  3. Map that index to the active locale's weekday label in full or abbreviated form.
  4. Count the day-of-year position and determine leap-year status from the same Gregorian year.
  5. Keep the scope honest: the arithmetic is consistently Gregorian and does not try to model local historical adoption dates.

Frequently Asked Questions

01How is the weekday calculated?
The calculator uses a standard Gregorian day-of-week formula. It combines the year, month, and day into a weekday index, then maps that index to a localized weekday label.
02Does switching to abbreviated output change the answer?
No. The math does not change. The switch only changes whether the result is shown as a full weekday name or an abbreviated weekday name.
03Are leap years handled?
Yes. Gregorian leap-year rules are built into both the weekday calculation and the day-of-year output. Years divisible by 4 are leap years, except century years not divisible by 400.
04Does this model country-by-country calendar history?
No. The tool applies Gregorian calendar arithmetic consistently to the entered date. It does not model when different countries adopted the Gregorian calendar.
05How is this different from Date Difference, Weekdays Calculator, and Age?
This page answers one question: which weekday does one date fall on? Date Difference measures the span between two dates, Weekdays Calculator counts selected weekdays across a range, and Age turns a birth date into elapsed age.

All calculators