Day of the Week Calculator with Date Facts
Find the weekday for any supported Gregorian date and report ISO week number, day of year, quarter, leap-year status and signed difference from a reference date. A separate arithmetic check validates the weekday.
Calendar date
Weekday and calendar details
day of the week calculator: formulas, interpretation and worked use
This day of the week calculator reports more than a weekday name. ISO week-year boundaries, leap years and signed date differences are calculated explicitly.
Transparent calculation steps
The day of the week calculator displays the intermediate values used in the result so users can audit the calculation rather than receiving an unexplained answer.
Input validation and edge cases
Impossible, incomplete and non-finite inputs are rejected with a specific message. Boundary cases are handled explicitly instead of being silently rounded into a misleading result.
Units and precision
Where units apply, inputs are converted to a consistent internal basis before the formula is evaluated. Results retain enough precision for checking and are then formatted for practical reading.
Worked interpretation
The result panel separates the primary answer, supporting quantities and a formula trace. This makes the calculator suitable for checking homework, planning scenarios and comparing alternatives.
ISO week rules
ISO weeks begin Monday, and week 1 is the week containing the year’s first Thursday.
Leap-year rule
A Gregorian year is a leap year when divisible by four, except century years not divisible by 400.
Gregorian calendar calculation
The day of the week calculator validates the entered Gregorian date and derives its weekday, day of year and ISO week information. Leap-year rules follow the Gregorian convention: years divisible by four are leap years except century years not divisible by 400. This correctly distinguishes dates such as February 29, 2000 from the invalid February 29, 1900.
Calendar-system limits
The day of the week calculator uses the proleptic Gregorian calendar for supported years. Historical locations adopted the Gregorian calendar at different times and sometimes skipped dates during transition, so a computed weekday may not match the locally observed historical calendar before adoption. ISO weeks begin on Monday, and week 1 is the week containing the year’s first Thursday; therefore late-December dates can belong to ISO week 1 of the next ISO year.
Which calendar is used?
The calculator uses the proleptic Gregorian calendar for all supported dates.
Why can ISO week year differ from calendar year?
Dates near New Year can belong to the first or last ISO week of a neighboring year.
Is the reference difference signed?
Yes. A negative value means the selected date is before the reference date.