Live math canvas
Weekday Calculator: Friday. 2026-05-15 is Friday. ISO weekday number 5 where Monday is 1 and Sunday is 7; day 135 of the year.
Formula applied
CalculationTime keeps the method visible so the number can be checked instead of blindly trusted.
Weekday = day name of the entered calendar date evaluated at UTC midnight in the proleptic Gregorian calendar.Your live breakdown
These values come from the controls above and update when the calculator changes.
Resulting answer
2026-05-15 is Friday. ISO weekday number 5 where Monday is 1 and Sunday is 7; day 135 of the year.
Assumptions used
Weekday only. Holidays, business-day rules and local deadline cutoffs are separate layers.
Master’s Tip
Master’s Tip: for birthdays, schedules and school planning, the Gregorian weekday is usually enough. For historical records before local Gregorian adoption, check the calendar system used in that place and year before quoting the weekday.
Printable record
Save the inputs, result, formula, assumptions, page URL and date together so the calculation can be reviewed later.
What-if check
Moving by seven days lands on the same weekday, while one-day moves prove the local cycle around the selected date.
| Check | Value | Note |
|---|---|---|
| -7 days | 2026-05-08 | Friday |
| -1 days | 2026-05-14 | Thursday |
| selected date | 2026-05-15 | Friday |
| +1 days | 2026-05-16 | Saturday |
| +7 days | 2026-05-22 | Friday |
Visual proof
2026-05-15 is Friday. ISO weekday number 5 where Monday is 1 and Sunday is 7; day 135 of the year.
This calculator tells you what weekday a date falls on. Enter a date, and it checks the calendar pattern to answer Monday, Tuesday, Wednesday and so on.
CalculationTime determines the day of week for a Gregorian calendar date by evaluating the entered year, month and day as a date-only value and returning the corresponding weekday label.
Weekday = day name of the entered calendar date evaluated at UTC midnight in the proleptic Gregorian calendar.
For 15 May 2026, the calculator creates the UTC date 2026-05-15. The calendar weekday for that date is Friday, so the result is Friday.
Master’s Tip: for birthdays, schedules and school planning, the Gregorian weekday is usually enough. For historical records before local Gregorian adoption, check the calendar system used in that place and year before quoting the weekday.
The basis is transparent general calendar arithmetic using numeric year-month-day inputs and UTC midnight. No legal deadline, holiday or regional adoption rule is claimed.
Methodology & Accuracy
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Weekday = day name of the entered calendar date evaluated at UTC midnight in the proleptic Gregorian calendar.
The basis is transparent general calendar arithmetic using numeric year-month-day inputs and UTC midnight. No legal deadline, holiday or regional adoption rule is claimed.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master’s Tip: for birthdays, schedules and school planning, the Gregorian weekday is usually enough. For historical records before local Gregorian adoption, check the calendar system used in that place and year before quoting the weekday.
Create the calendar date from the year, month and day, then read the weekday assigned to that date in the Gregorian calendar.
UTC midnight keeps the calculation date-only, so daylight-saving time changes do not shift the result by an hour.
Use caution. Many places used Julian or local calendars before adopting the Gregorian calendar, so historical weekdays may depend on the calendar system in use.
No. It only returns the weekday name. Holidays, observances and business-day rules are separate calendar layers.
Print the entered date, weekday result, Gregorian-calendar basis, UTC-midnight method, historical-date caution, page URL, generated date and notes area so the record can be checked later.
Weekday calculation connects ordinary scheduling with calendar history. A day name looks simple, but it depends on a shared calendar system, the seven-day week and the convention used to map numbered dates onto weekdays.
Weekday calculation works because weekday names repeat in a fixed seven-day cycle. Once a calendar date is mapped onto that cycle, moving forward or backward by seven days lands on the same weekday.
Modern civil dates usually use the Gregorian calendar. Leap-year rules keep the calendar aligned with the solar year, which is why weekday arithmetic must account for month lengths and leap days rather than simply counting every year as 365 days.
Different countries adopted the Gregorian calendar at different times. A printed historical date can therefore have a different weekday depending on whether the Julian calendar, Gregorian calendar or a local convention was used.