Weekday Finder
Find out what day of the week any date falls on.
How weekday calculation works
JavaScript’s Date.getDay() method returns the day of the week as a number from 0 (Sunday) to 6 (Saturday). The underlying calculation uses the Doomsday algorithm or Zeller's congruence — methods that compute the day of the week for any date from a mathematical formula rather than counting through every calendar.
Zeller's congruence (simplified):
Day = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7
Where: q = day, m = month (March=3, adjusting Jan/Feb), K = year%100, J = century
Result: 0=Saturday, 1=Sunday, 2=Monday … 6=FridayFun calendar facts
Week numbers explained
There are two common week numbering systems and they often give different results for dates at the start and end of the year. The ISO 8601 standard (used across Europe and in business internationally) defines week 1 as the week containing the year's first Thursday. The North American system simply counts weeks from January 1.
| System | Week 1 starts | Used in |
|---|---|---|
| ISO 8601 | Monday of the week containing January 4 | Europe, international business, ISO standards |
| North American | First Sunday on or before January 1 | US, Canada, Latin America |
| Middle Eastern | First Saturday on or before January 1 | Some Gulf states |
Frequently asked questions
How do I find what day of the week a date falls on?
Enter any date and the tool returns the weekday — useful for historical dates, future planning or settling a "what day was I born" question.
Does it work for past and future dates?
Yes. It works for any date in the standard Gregorian calendar, past or future.
How is the day of the week calculated?
It uses the calendar date's position relative to known reference points, the same logic browsers and operating systems use, so it accounts for leap years automatically.
What calendar does this use?
The Gregorian calendar, which is the international civil standard. Dates before its adoption in 1582 may differ from historical records that used the Julian calendar.
Uses JavaScript’s Date object. Week number = weeks since January 1st of that year.